Yoga requires no flexibility to start, as the above poster was saying, but you will quickly develop it. A big part of yoga is taking your ego out of it though and being ok with where you body is at, at any given time.
I saw a completly out of shape 56 year old man who couldn't touch his toes or sit cross legged on the floor learn to do both, and more, given time, patience and an understanding that it was ok that he couldn't do it.
Going to a beggining class is the best idea. Don't just throw yourself into it. You need someone knowlegable who can give you corrections and modifications to poses. After you've learned how to do some safely you can begin practicing at home.
Good luck. Yoga has definitely changed my life and for the better. It is a lifelong journey, and one of the best things I have ever done for myself.
Lokah Samastah Sukhino Bhavanto
'May everyone everywhere be happy
May the whole world be joyous'