I just wanted to run through what tonight's possible scenarios would mean:
1.) Hillary wins both Ohio and Texas. I don't see how this doesn't send us into a six-week-long battle for Pennsylvania. Yes, Obama is still going to lead by more than 100 pledged delegates at the end of the night. And, yes, it's not clear where Hillary makes those delegates up. But there's no way Hillary exits the race after winning two populous states, regardless of how narrow the victories. If nothing else, she'll have a huge media afterglow to bask in. And, in effect, the voters will have said they want the race to continue. I don't see the notoriously weak-kneed superdelegates contradicting either group.
On top of everything else, the Clintonites will point to their Texas primary victory and a (likely) Texas caucus loss as evidence that caucus states are screwy and should be viewed skeptically. It's much too convenient an argument for them. But the starkness of the Texas example will give the media pause.
Of course, if Obama can come back and win Pennsylvania in late April, he ends the nomination fight right there. Not only would the delegate math clearly be impossible for Hillary, so would any appeal to populous-state wins. The superdelegates would almost certainly abandon her. She has to keep winning to get this to the convention.
2.) Hillary wins Ohio; Obama wins Texas. Hillary's inclination will be to fight on (witness her recent remarks about "just getting warmed up"). And the press will let her do it for a couple of days. But i just don't see how she sustains it. Obama's been picking up superdelegates at a steady clip lately. I'd expect that to continue under this scenario, perhaps even accelerate. You may even see party elders issue dire warnings about her damaging Obama for the general.
Also, don't forget the money. Obama probably raised well over $50 million in February, but the campaign's been mum so far about the specific number. As some of our commenters have pointed out, they may be saving the announcement for when it can finish Hillary off once and for all. On the heels of a Texas win, $55 million or whatever would go a long way toward convincing the media and wavering party elites that Obama has this thing wrapped up.
3.) Obama wins both Ohio and Texas. I see almost no chance of this happening after NAFTA-gate. But, as Mike says, if Hillary somehow comes up short despite the gale-force wind at her back, I'd expect her to concede tomorrow night. Bill said she had to win both states to survive. You can try to spin a close loss in one, but no amount of spinning will convince the media and the superdelegates she deserves yet another firewall.
Update: A friend e-mails to remind me not to forget about Vermont and Rhode Island, where one candidate's net delegate lead could be larger than in, say, Ohio. If Hillary's on the wrong end of those outcomes (Rhode Island is close-ish and Vermont should go lopsidedly for Obama), and loses the delegate race in Texas, she could find herself in a bigger delegate hole than when she started the day.