For Mitt Romney, very little changed today. He can continue to promise in his stump speech to repeal the Affordable Care Act, which President Obama will probably continue to remind people was largely modeled after Romney's own Massachusetts health care reform. Romney is going to have to come up with some kind of an alternative to the plan, though; he's not going to be able to just complain about it endlessly through November. For right now, though, the Romney campaign is probably going to raise a whole lot of money from angry conservatives who want to see Obamacare destroyed.
For President Obama, it's a little more complicated. The Affordable Care Act is still unpopular with the American people when considered as a whole, even though when considered individually, all of its major benefits are wildly popular. Part of Obama's job this fall is to convince people that this is a good thing for them. SCOTUS calling it a tax isn't going to help the Obama campaign, though, and there are tons of videos floating around out there of President Obama telling people the Affordable Care Act isn't a tax. But a win is a win is a win, and this decision does validate a huge part of the president's accomplishments, and it also heads off a fledgling conservative talking point that the Obama administration spent months on shoddy legislation. On the whole, this is a best-case-scenario for the Obama campaign, though I expect the Romney people will raise way more money on today's news.