It's known that you can share Your Stage creations via RaceNet, and I expect that you could use them in leagues. I have heard some talk about live PvP stages - maybe not for Your Stage - but as I understand it, Codemasters have some concerns about its implementation.
I will answer my own question. I did some digging and the info is in the release announcement and then they shed more light on it in an EU PlayStation Q&A post. Some snippets:
"So, this is big: Your Stage technology also provides you the tools to generate your own stages. At the push of a button, you can specify what kind of stage you want – location, long, short, complex, straight-forward, night-time, sunset, rainy, whatever – the choice is all yours. And once you hit that button, you have a completely unique stage created to your own personal spec. This is a stage you’ll be able to take time and time again, share with your friends and the DiRT Community, and even use it with league and PvP functionality. And with over a billion different combinations of stage at your disposal, the possibilities are near endless. It’s absolutely mega, and I don’t use that word lightly."
"Beyond Your Stage we’ve been working on a whole suite of improvements across the game. In rally these include things like PvP support, multi-class rally events and flying finishes (where you cross the line and then have to stop at the time control)."
"One of the most requested features for DIRT Rally was PvP, and we’re bringing it back in two different flavours. You may remember Jam Session from DiRT 3, which makes its triumphant return to provide you with all of your custom lobby needs. Also back is Pro Tour, powered by a new, truly competitive ladder and ranking system."
"We have an almost infinite amount of rally stages available in the game with our new tech, which means players won’t ever be able to learn a stage off by heart – especially when it comes to our online events. And then bringing the 8-player MP Landrush, PvP Rally, Rallycross MP, and the ranking system... there are so many reasons for people to return to the game time and time again."
"PvP Rally is in DiRT 4! There were limitations when it came to DiRT Rally, so it’s something we prioritised with DiRT 4."
"The short answer is yes – PvP Rally is returning, so you and your buddies will be able to jump into a session together and race against one another in real time. It’s one of the things players asked us most for throughout the development of DiRT Rally, so with DiRT 4 we’ve made sure to deliver."
So there you have it. It was confirmed way back in the release announcement. Don't know how I missed it. I also read on other blogs that the system will allow you to save PvP rally formats, so in a session you evidently could quickly swap locations and load full rallies in the lobby with the push of a button. Gonna be EPIC!
http://blog.codemasters.com/dirt/01/dirt-4-road-book/
http://community.eu.playstation.com...-A-with-Codemasters-The-Answers/td-p/25037317
[EDIT] Forgot to mention there will be no ghost cars in PvP. That's fine with me, I turn them off in Loeb as I find them distracting. I assume they will show gaps at the split points or maybe have a progress indicator or something. [/EDIT]