I'm not sure but I think this one may be specific to the Mac. Still, thought it was worth mentioning.

Recently I bought a Skype kit and decided to have another go at it, having use and left it before. I opened up Skype, tried to login and recieved the error: "Unable to mount database". A quick Google yielded some results; this posting on the Skype forum was the best. Basically, the guy tells you to:

1. Go into <Your home directory>/Library/Application Support/Skype/ in Finder
   
2. Delete the folder named after your Skype username.
3. Close Skype 
4. Open Skype
5. Log in as normal

That should solve the problem; at least it did for me. Some of the other information I came across pointed to some kind of corruption of a profile file in that directory that then caused Skype to throw a wobbly for that particular user. Looks like pretty poor programming on behalf of Skype, though. Us Mac users aren't usd to uninformative errors...