If you upgraded your Safari to the new 3.1 version, use Gmail, and now your Shift or Tab keys are making your cursor jumping back to the To: field, here is the solution: change your default language to English (US); click on "Newer version" on the top right of the screen - only English (US) version has this option enabled; you are done, Gmail should be working perfectly now.

I guess people at Cupertino doesn't use Gmail...

Update: the same problem happens with Gmail for Google Apps, and I don't know how to solve it. Need a hand.