Could be. The "staunch ally" of the United States has told its troops "to open fire" if US troops launch any more raids across the border from Afghanistan. I'd imagine this is a serious corner for them to be turning.
Will this mean war? Given that Pakistan has never been very forthright about tracking down those al-Quaida or Taliban fighters who are hiding within its borders, and that the US has good reason to believe that there are plenty such fighters hopping back and forth across the border to attack them, I'd say it would be pretty stupid to imagine that there couldn't possibly be war between the US and Pakistan.
Christopher Hitchens says that Barack Obama is serious about dealing with Pakistan. I don''t' know if it's fair to say "only Obama"- I'd imagine that McCain would also deal with Pakistan, although the US has pussyfooted around Pakistan for quite some time now. Bernard-Henri Lévy's book "Who Killed Daniel Pearl?", which I talked about here a few years back, made the same point about the ISI, the Pakistani secret service, being largely sympathetic to the Islamic terrorist organizations: "the most rogue of the rogue states", as he put it.
So, war with Pakistan seems more plausible than war with Russia, for example. And, actually, it makes a bit more sense than war with Iraq.