Along with Wisconsin, Pennsylvania and Michigan make the most tempting targets because they're all blue states which (temporarily, we can pray) are completely controlled by Republicans, thanks to the 2010 wipeout. Virginia's also a possibility, but it remains in play for the GOP on the statewide level, so Republicans might continue to prefer winner-take-all there. (After that, you're talking about Ohio, Florida and North Carolina—states the GOP almost certainly doesn't want to carve up.)
But it's far from automatic. BeloitDem points out that Republican margins in the state legislatures in MI, PA and WI are fairly tight and it wouldn't take many defections to derail this scheme. (In Virginia, it would take just one: Lt. Gov. Bill Bolling, who casts tiebreaking votes in the evenly-divided state Senate. And he's pretty incensed at the establishment these days.)
The reason this very same scheme ran aground in the Keystone State is instructive, too: Republican members of Congress were extremely wary of turning their individual turfs into national battlegrounds every time a presidential election rolled around. GOP leaders in Pennsylvania are therefore pushing an amended plan now, one that would award electoral votes based on the total popular vote share in the state. This method would have given Obama 11 electoral voters and Romney 9 in Pennsylvania, instead of the full 20 being awarded to Obama outright. And of course, this plan would also damage Democratic chances at the White House in the future because similar changes obviously won't be made in large red states like Texas.
This modified Pennsylvania approach worries me the most, since it'd probably be the easiest to pass. Democrats therefore need to keep pressure on wobbly Republican lawmakers in each of these states—and to make as big a stink as possible about how shameful these shenanigans are. I'll bet newspaper editorial boards won't like these hijinks one bit, and I think ordinary voters can be convinced that the GOP is playing political games with their right to vote.
What's more, changing the system will also jeopardize the swing state status of each of these states. Under the new PA plan, no one would bother contesting the state as you'd need to win 56 percent of the total statewide vote just to move the needle from an 11-9 split to 12-8. No one's scored that high in Pennsylvania since the Nixon landslide of `72—not even Obama in 2008—so the state would go from a frequent battleground to an afterthought. To be clear, I don't care for the electoral college one bit, but if appealing to any sense of "swing state pride" helps scuttle these efforts, I'm all for it.