Joined: 21 Jul 2006
Location: London, UK
You've got a duplicate font on your system I think. Check the following locations:
In this folder there will be a single Arial font. Delete it. Then restart your system. The reason this happens is to do with upgrading from Leopard to Snow Leopard, older version of the Arial font (which is broken as far as I recall) gets copied to this location during installation. The user-area locations always over-ride the system level locations... so programs start attempting to use this corrupt font, and you get the problem described.
Let me know if that doesn't fix it.