If you're using IE9, it will randomly decide loading the Lato fonts we use is bad, and default to something else.
If you're using FF with Firebug (or other dev tools), recently one of those plugins broke the font load for whatever reason. It happened a while back, got patched, and then started up again, though today it doesn't appear to be happening. Probably not limited to dev-related plugins, but since we use a bunch of those here, that's what we noticed had the problem.
As for Chrome, we haven't run into any font-related things with that browser, though it may have plugin--related issues like FF above.
If you're using Opera? Opera... wat r u doing? Opera... stahp!
And finally, if you're using Windows, there's Cleartype Font "things" you can do with the various versions to address handling of Cleartype fonts. If you search for (windows version) font smoothing, you'll probably find relevant info/instructions. There's also a Microsoft-issued 'ClearType Tuning' program for Vista (and probably 7).