Yes, I started a whole new thread about LSDs, even though there are already a number on the subject. I didnt want this to get buried in page 17 of a long thread that hasnt proven definitive. For example, the Initial Torque thread hasnt really provided anything conclusive in my opinion. Maybe this thread wont either, but I would hope we can post more of what does the LSD do in this game rather than a bunch more formulas, theories, tested it on one car and of course the handful of know-it-alls who have it figured out, yet just can seem to provide a simple explanation. What does the diff do in this game, from a programmer point of view? Move a setting up or down, does the car turn better or understeer (not, it locks to 95% rather than 90%). I dont know if this community can do that, but lets give it a shot.
This is what I have found the diff to do in GT5.
Initial Torque:
Lower number, a more open diff = better turning /more oversteer
Higher number, a diff closer to lock = more planted/more understeer
Decel/Accel:
These need to be tuned together. This is how you balance the attitude of the car. Decel affects braking through apex or just before throttle application. Accel affects from apex or throttle application through corner exit. You should use Accel/Decel to balance a car all the way through the corner it should be smooth, not push in, understeer out or the opposite.
Decel:
Lower number, a more open diff when off throttle = better turning /more oversteer when off throttle
Higher number, a diff closer to lock when off throttle = more control /more understeer when off throttle
Accel:
Lower number, a more open diff when on throttle = better turning / more oversteer, and more possibility for inside wheel spin when on throttle
Higher number, a diff closer to lock when on throttle = more planted / more understeer and less possibility for inside wheel spin when on throttle
So how do you choose the settings? It all depends upon the power of the car and your driving style. The goal for tuning the LSD is to find a balance of understeer/oversteer and to reduce inside wheel spin. It doesnt need to be any more complicated than that. So where should you start? There are many ways to go at this, so I will just describe how I approach it. Others will have methods to achieve the same goal a faster, better driving tuned diff. I start with Initial at the default of 10. I truly believe that you have to balance the car first with Decel and Accel, then return to Initial.
My steps:
1.) Initial at 10
2.) Tune the balance with Decel/Accel. I just recently found a track to tune on that really exposes the weaknesses of an LSD tune Grand Valley East, reverse. The first two turns at the end of the long straight really test the Decel settings of the diff. The first corner is a mid-level left hander, followed immediately by a sort of tight left hander. You can carry some speed through the first corner, but will need to brake a bit for the second all while the car is off balance, mid corner. If your car has understeer through this section, lower the Decel. If it has oversteer and tries to spin out trying to enter the second corner, raise the Decel.
The other section of Grand Valley East, reverse that exposes a weak LSD tune is the 180 hairpin just before the long straight. Decel can be tuned here too, but the settings that work on the other end of the track will also be strong here. Focus on Accel from power on through exit. If the inside wheel spins or the car has oversteer, raise the Accel. If the car has understeer, lower the Accel.
3.) Now tune the Initial Torque. This is simply an oversteer/understeer proposition times power. For lower powered cars, you can run very low numbers (between 5 and 12). For mid-powered cars I tend to use ranges from 8 to 15. I havent found any gains for low to mid power cars to run super high initial torque numbers. These cars just dont put out enough power to take advantage of a stronger diff. For high powered cars, the game changes, depending upon the car. I have used numbers between 10 and 40. It truly depends upon the attitude of the car and how much power and downforce it has. High powered cars also need more throttle control not just slamming back to full throttle at the apex. More on this below.
4.) Return to Decel/Accel and fine tune, if needed. Once the Initial is set closer to the liking of the car and your driving style, go back to paying attention to the Decel/Accel settings. You may find that corner entry could be a little bit better raise or lower Decel one or two numbers. Or that corner exit could be a little better raise or lower the Accel one or two numbers. Finally take one last look at Initial and begin to adjust up and down a number or two, looking to confirm that your LSD is optimized.
One last item to be addressed is outside wheel spin. I will admit that this is the one thing that I have not been able to tune out with the LSD. It challenges logic to how any LSD should work (more locked, less inside wheel spin less locked, more chance of inside wheel spin). In GT5, I find that when you can move a setting from minimum to maximum and see little difference in affect and/or lap times, the adjustment you are trying to make is not the adjustment needed to solve the problem. You are asking it to solve something that it cannot. My hypothesis (this is the only place in my LSD tuning that is a theory) is that outside wheel spin is more affected by downforce, suspension settings and driver inputs. The outside wheel is being pushed beyond its available grip. Either slow down a little more or apply gentler throttle inputs or change your suspension settings to give more grip on that end of the car. Take the Yellow Bird or the Viper ACR to Grand Valley East, reverse and turn the Initial and Accel all the way up. How can these cars still spin only the outside wheels with an almost locked diff? Both should spin in that situation. Therefore, I believe that getting rid of outside wheel spin is not the job of the LSD.
Your thoughts? I ask again, what does the diff really do in this game?
Never before have I have so much trouble tuning anything but LSDs in GT5 have me stumped! 
