I do a lot of Ruby on Rails at work these days. This weekend, I'm getting some stuff done from home, so I tried to run a Rails app on my laptop... And it stalled/hanged indefinitely, using the processor flat out.
It turns out that I hadn't used it since I upgraded to Snow Leopard. This seemingly broke the installation a bit. Initially I tried just removing the offending gem (sudo gem uninstall mongrel), but when I re-installed it, the problem persisted.
sudo gem uninstall mongrel
sudo gem uninstall fastthread
sudo gem install mongrel
It turns out that the fastthread library I had installed was the problem; I suspect that it was trying to load 32bit native code into my 64bit ruby.
Anyway, hope this is helpful to someone.