Here’s an experiment:
1) Sign up for a basecamp account at 37 signals.
2) Close your browser
3) Start over at 37signals.com, and try to figure out how to log into the account you created.

I tried this – I can’t.

From what I can tell, the only way you can ever get to the account you created is to save the e-mail they send you when you sign up, and bookmark the link. The link, amazingly, isn’t even on the basecamphq.com domain! These guys write so much about usability, but this incredibly simple task can’t even be done on their website.