In reply to ksjs:
When you stretch make sure you are stretching BOTH muscles in the calf. Stretch with a straight leg, AND a bent leg. Its likely that its your soleus (deeper) causing the issues, not the Gastroc (more superficial).
Have a quick look at this
http://globaltherapies.blogspot.com/2011/07/calf-stretches.html It might help.
As the other guys said, make sure youre hydrated, don't go at it too heavily or too far at once, also, if youre using old screwed up shoes, they might not be helping you out either.
Also, if you run more frequently, but for less time each time, you will get used to the exercise, and will be able to increase the duration more quickly (as opposed to hurting so much that you start to decrease because of the pain)