After further debugging it appears to be related to FastCGI. I could run my site fine with WEBrick and even under (Slow)CGI, which is how it is running right now.
It's been over 5 days and this still hasn't been fixed. The turnaround time on DH's customer support seems to be consistently hovering at 2 days. I write them asking for help, they write me two days later telling me to check out the "rails gotchas wiki", I write them back telling them I've already done that, two days later they write me back saying "ok fine, I guess we'll try and help you".
Frustrating to say the least.