I have to agree, yoga is the best way to improve those hamstrings. After 22 years of running (I am 42), and not being diligent about stretching afterwards, my hamstrings and lower back are incredibly stiff and store. After doing yoga 2 times a week for 6 weeks, and just screaming through some of those poses, I do see incremental improvements. Give it a try.
I have found that Bryan Kest's DVD with 3 combo's (beginner, intermediate and advanced, I am already on the intermediate) and the MTV yoga/power yoga (it is a 4 pack with 2 pilates DVD's) are great ways to start. Shiva Rea's Shakti Yoga has a great yoga matrix you can choose from, but it is not for beginners. She is so flexible, where does she come from? I found all these dvd's on amazon.com (no stores on my little island). Just go to amazon and type in yoga dvd's. Peggy somebody or other (Cappy?) does a Yoga For Inflexible People, which I refused to admit I needed.
Good luck, and remember that the flexibility comes in teensy tiny steps.