It's not exactly something that body-builders can show off, but it's common trouble maker.
http://www.exrx.net/Muscles/Iliopsoas.html

It's notorious for getting short and tight in people who spend a lot of time sitting, causing back pain.

Here's an online article from Yoga Journal about psoas stretches. (You don't have to be a yoga expert to do the stretches.) The best one is the one they aptly describe as "The Ultimate Stretch." http://www.yogajournal.com/practice/169_1.cfm

This site has a bunch of good stretches for hip area muscles:
http://jaxmed.com/massage/pirformis_stretches.htm

Go easy and work at it consistanly, a little time every day. You'll get results. It's better to stretch warmed-up muscles, you'll probably get better results after your ride.

Good luck!