From time to time, Pandora makes an API change that breaks Pithos. As an unofficial client, Pithos has no prior notice about these changes.
2012-04-24 (bug #988395)
Versions 0.3.16 and previous are broken by a Pandora update.
Version 0.3.17 fixes this issue. If you installed from the PPA, update Pithos through Update Manager. If you installed from the Ubuntu repositories or are using another distribution, follow the install instructions on the home page.