Mike,
Take the seatpost out and look down inside the seat tube and see if the bottom of the tube is corroded. If it is, then the bottom section of the tube can simply be cut out and replaced. This was a fairly common occurence on mountain frames and I did several tube-ectomies.
If the tube is not seriously corroded, the crack can be stop-drilled and welded or brazed. It would be prudent to put a small reinforcement patch over the repaired area.
None of these are a cheap fix, of course, and both would require re-painting.
Another suggestion would be to just stop-drill the end of the crack with a 1/16" (1.5 mm) drill and wrap the cracked area with tape. Remove the tape and inspect after riding to see if the crack propagation has been halted. If you ride in wet conditions, remove the seat post after each ride and dry the inside of the seat tube. Coat the interior of the tube with some good grease to increase the corrosion protection.
Good Luck and keep me posted
Scott Bengtson