I have a single gnome panel at the top of my screen that holds all the applets I need. It started taking ages to appear (tens of seconds) after logging in. I tried deleting various applets in the hope that one of them was responsible for the issue but nothing worked.
Some Googling lead me to Bug #593226 in Launchpad. I opened the Passwords & Encryption tool (in Applications > Accessories) and found that my passwords included two for Desktop Couch. I deleted both - problem solved.