I am having a frame warrantied for this exact issue. In my case, an excessive tolerance between post and seat tube caused a crack at the bottom of the split near the top of the seat tube. I found that if I removed the seatpost clamp, and inserted the seatpost, it would drop freely into the seat tube, and still have noticeable play at full insertion. This is not the case on my other frames.
For reference, my 30.9 posts(used a KS and a Spesh) measured 30.84(KS) and between 30.80 and 30.83(Spesh had a slight taper). This using a calibrated digital caliper w/.02mm accuracy and .01mm repeatability. With either post fully inserted, there was a gap at the bottom of the split at the top of seat tube. i could insert a dental pick into the gap between tube and post. You may want to check this. If there is a gap in between the post and tube, you may experience a similar frame failure later on.
Apparently the honing process used to clean up the seat tube isn't so precise.
Tom Robbins wrote:It's never too late to have a happy childhood.