Why can't I type lower case 'a' in Safari when entering text in a field?
I had exactly the same problem, and after disabling extensions and restarting Safari it went away.  I then looked more closely and found that I had set the letter lowercase-a as the keyboard shortcut to allow flash in ClickToFlash.  Apparently this made it not work as a character in any text boxes in Safari!  I now have all the extensions back on but without that key shortcut, and everything is working fine!

BTW, this is clearly a bug in ClickToFlash, since the default key to make flash full-screen is lowercase-f, but I can type that fine in any text box.