They hinted at cross-system play being a thing when they announced this game months ago. But at E3 they said there will be levels (and possibly characters) exclusive to each system. So maybe it's only possible with shared elements?
Not sure how to feel about that. I guess that means Wii owners will be able to fight against the 3DS characters, but only if someone is playing the 3DS version and selects them? Honestly, if they're going to make people buy both games anyway, I hope they at least let you unlock those characters on the other version if you own both. I wouldn't want to put down my controler and go play a 3DS just to switch to the character I want to be.
All that said, I'm really glad they went for two distinct visual styles on the two consoles, instead of just one being a pixelated version of the other.
I'm really more excited for the Yellow Devil teaser at the end there (and a nod to Guts Man is always nice), but I have to wonder how many humorless dorks are going to buy this game now just as some delusional spite against Street Fighter X Tekken.