I actually just saw this, and it's something I've been talking about with MD friends of mine for a while now.  As a matter of fact my ecology professor's final lecture in April of 1999 addressed this very likelihood and I've attempted to be somewhat evangelical about it. Like all big problems, it's multifactorial in nature.  For one thing, we could be looking at this scenario whether we were experiencing global warming or not.  The thing is, bacteria have a stiffer tolerance for temperature extremes than eukaryotes--we're fortunate that we don't get to notice that as often as we could.  Cholera was pandemic in Britain and France for several decades in the 19th century, for example.  Clearly it handles cold winters, but it doesn't handle modern sanitation methods to which wealthy nations have access.  Admittedly, viruses and eukaryotic pathogens aren't so hardy, but all they require is a chance.

Where global warming could massively change the game is in the vectors.  The fleas, the ticks, the beetles, rodents, the copapods, you name it, were already getting a little bit more travel due to globalization--which may be the bigger threat in the short term, in fact.  But if we begin to experience wetter and more tropical summers in the temperate zones, the animal vectors won't need as much help from humans for transit.  They'll move into the new niche just as soon as it gets tolerable for reproduction.  Our respite is the fact that even in a post-global warming world, those of us above the tropics will still have seasons, and quite likely more extreme ones at that, which will cut out some of the troubles from the vectors, at least for part of the year.  Slim comfort, especially when we consider the possibility that the tropics themselves will be less comfortable for humans than they are now.  Emigration will turn humans into vectors.  

I can easily say I've personally experienced this first hand. West Nile disease used to never be able to get a foot hold in Canada due to our cold weather. Now it's here in force, as the cold has dissipated significantly. Still, it's not like hundreds of people are falling over dead. We'll adapt as we always have.