I'm annoyed by this Safari bug all the time, thanks for pointing out pinch zoom, never considered that.
Afaik, there is no support for pinch zoom in Safari, certainly pinch zoom has never worked in Safari v5.1.3 or earlier on Lion. I therefore suspect that pinch zoom has activated despite not being correctly supported in Safari.
If so, you might fix this Safari bug by disabling pinch zoom completely in System Preferences. I'll try this myself and report back.
I've found that pinch zoom works well enough in Preview, but I'll happily sacrifice that functionality to fix Safari.
I have never managed to fix this by unzooming as Matt Love suggests, certainly the command and + or - keys don't fix it, nor does double tapping, nor does pinching or unpinching.
Update : It appears disabling pinch zoom fixes this problem. As I said, Safari apparently never support pinch-zoom anyways, but this disables pinch-zoom in preview. Anyone know if pinch-zoom can be disabled for specific applications?