Skype for Mac gets a Snow Leopard hotfix
This does not automatically come up, BUT when you manually look for the update Skype > Check for update you may be faced with a message “unable to mount database”, or “incorrect password”.
You may get the report that Skype reports that it closed unexpectedly and if you wish to reopen. If you do so I believe this is one of the causes for this issue.
Close Skype completely and open again. You should then be able to log in with no trouble. If not then remove the application and reinstall again.