I discovered something interesting today: FireFox 3.0.X periodically reloads your hosts file while running.
I'd been using IE7 and Chrome when working with my code recently, but today I'd had a Chrome specific bug and fired up FF as a double check. While it was running I needed to view something on the unmodified version of a URL I'd overridden, so opened my hosts file, updated it and then launched a new browser to check out the other version. After about 5 minutes I refreshed the FF page, and suddenly I get a warning that the file doesn't exist. At first I thought I'd deleted something, but it turns out that FF update from the host and was now requesting the site where the file didn't exist, instead of my development version.
I reverted my host file and refreshed for a bit, and sure enough, suddenly it switched back to the development version. As far as I'm aware, FF3 is the only browser that updates its host file while running; every other browser seems to only load it on start and then to hold onto that result.