The problem and the fix.
Ever since 37signals introduced the launchpad and unified account management for all of their products, I couldn't login to Basecamp or Campfire with Fluid. I didn't have much success finding help via Google, but I finally stumbled upon Steve Sanderson's fix today: simply add *launchpad.37signals.com* including the asterisks to your whitelist of URLs in Fluid (it's in Preferences > Advanced), restart Fluid and you're up and running.
What is Fluid anyways?
Fluid for Mac OS (Windows users have an alternative with Prism) allows you to "Turn your favorite websites into Mac desktop applications". Basically you get the website inside a desktop app, complete with a dock icon. You could go crazy and make apps for GMail, facebook, flickr, last.fm, your feedreader, ... — personally, I only use it for Fever, Basecamp and Campfire.
Thirtyseven signals and a basecampfire? Are you nuts?
37signals are creators of fine web-based applications for groups and companies, trying to make work for the rest of us easier. We at webfactory use Basecamp, their most famous piece of software, to manage our projects (keeping track of to-do lists, messages, deadlines, progress and what-not).