*** IMPORTANT ***
Remember, if you're a VIP (purchased the special Limited Collectors Edition, or upgraded recently), there's supposed to be a 400 MSP discount for you--it will cost you 1200 MSP instead of 1600 (ooh, I'm a math wiz).
***However, YOU MUST purchase the pack via the marketplace WITHIN THE GAME to get this discount.
**Update -- According to the Forza 4 forums, it's only the VIPs trying to purchase the car pack within the game that are experiencing problems with Xbox Live Marketplace.
I was pleasantly surprised to get up this morning and see the Porsche Expansion Pack is available on XBox live...
but, yes, there's apparently a problem with XBox Live Marketplace for the moment. Guess I'll go ahead and start downloading the nearly 500GB Gran Turismo update while I'm waiting for this to work. Isn't it funny/interesting that Polyphony Digital once again releases a major update the same day something significant is released for Forza 4?
...Wow, the GT5 update is actually downloading REALLY fast for a change. Hopefully Xbox Live will work soon.
Got my in-game credits built up, and wanna start setting up my Porsches. Remember to buy and modify one Porsche first, and race it several times to level up the brand to get discounts or no charges for upgrades for the other models.