I will heartily second the advice to avoid static stretching before exercising, and stick with dynamic warm-ups instead. However, a person's flexibility is most definitely NOT a "pretty much set" quality, and if your goal is to increase flexibility, static stretches (say at the end of a ride, or the end of the day) can be an important part of that. When not riding my bike, I teach ballet and modern dance, and still dance and perform myself. I was never a naturally flexible person, but through diligent static stretching, I have achieved very good flexibility (by which I mean that other dancers are impressed by my flexibility, not just lay people.)

Whether increasing your flexibility beyond what you need for function actually improves performance on the bike or reduces injury is another topic, with inconclusive results that tend to point to an answer of "no."

I'll also put in a plug for Tom Danielson's book Core Advantage. I've been regularly doing some of the workouts in it, and they've been very useful.

Another great cross-training activity for the bike is ballet class. I realize a lot of the guys here might be intimidated by the idea, and dismiss it out of hand, but you should give it a try. It works the back, core, glutes, quads, hamstrings, and calves, develops balance and coordination, builds strength, especially explosive strength, and helps counteract the stress on the upper back and neck caused by road bike posture. (And to try to head some of you off at the pass, asking if you have to wear pink tights and a tutu is about as original as the driver of a pick up yelling "hey, Lance" and dieseling you as he drives by...)