For 2 days, some people are reporting login window disappearances. Happened to me on both my Windows XP and Ubuntu 10.10 machine. The solution is quite the same:
- Locate the Skype user settings folder
- Delete it
- Open terminal
- rm -rf /home/username/.Skype
- Start -> Run
- Select All (CTRL + A)
As some people pointed, there is a better solution than deleting the whole Skype user settings folder. Instead, you should only delete the shared.xml file in it.