It's happening. The two biggest names in fighting games are putting out 2 games together.
So the Capcom produced one might not have a tag system but you can definately call in help during ultras.
We'll make a second thread for Namco x Street Fighter if we need to.
This is all the news we have right now. More as things develop.
Update: According to eurogamer, the next big reveal will be at Captivate 2011. No word on what or when Namco will let slip on their version/