For some reason, the other day, out of nowhere, Google Webmaster Tools started showing all these errors while crawling my site, saying that it couldn't reach my pages. I checked my robots.txt and talked to my hosting provider and everything checks out, so I'm not sure what's wrong.
I'm seeing the same issue across multiple dedicated servers in different datacenters.
Matt Cutts posted on his blog that this looks like a Google issue. I wouldn't freak out unless it continues for another couple days.