Hi,
Is the Monaco font still present and enabled in FontBook?
Check if it's present in FontBook and that it isn't marked as requiring resolving (it will have a bullet point to the right of the name). If it does need resolving, use the Edit menu option of FontBook to resolve it.
If none of the above applies, one quick test you can do to help narrow down things is to try Safari in another user account. This will help us to know whether your problem is local to your account or system wide. If you don't have another account you can use System Preferences -> Accounts -> [+] to create a test one (and [-] to remove it if needed)