I've now been playing with Flixel and PyGame for a weekend, and I think I'm going to end up working in Flash for Ludum Dare. PyGame was great. I was able to get up and running quickly, and working in Python definitely matches my brain nicely. Flixel is just so fast to build things though, and the language feels like a type-hinted bastard child of PHP and Javascript, so I'm comfortable enough with it.
I did find some new awesomeness with tools, however. First, mtPaint is great for pixel art. Much easier than GIMP. Second, Audacity has a bunch of new stuff in 2.0 that makes generating sounds a lot easier.
Oh, and for developing in Python, I would now recommend Aptana Studio (with vrapper if you are like me and need vim keybindings). Wonderful integrated environment. I might start using it on Linux for my PHP development at work. Of course, FlashDevelop is great for Flash (under Windows or Wine).
Overall, I'm very happy with what I got accomplished. My PyGame source and Flixel source are on GitHub, and you can try the Flixel game I made from a tutorial. See if you can beat my high score of 80!