As far as I know, Valve is not on the vendor list for PAX and hasn't been since the year before Portal 2 was released. I can't say for what will be in the PC area, however, although since that has limited time slots my guess is that DOTA wouldn't be included for that reason. Perhaps one of the [E]'s who are working there can comment with better knowledge, however.
I really do hope that Dota 2 has a presence at least in the PC Area this year since they use steam to launch games and vlave has added local play since then. I heard that last year they didn't put it because of bandwidth (LoL used internet, don't know why Dota 2 wasn't there) though this year Dota 2 has local play now so that shouldn't be an excuse this year.
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Edit: At least Dota 2 presence in the PC Area this year?
PAX Prime Attendee since 2006, BYOC Attendee 2008-2012, Buttoneer 2010-2014
https://www.pinnypals.com/pals/alegria