Except it's not just Alloy that get's this, nor just the 29er's, but the whole anthem range (though it's mor pronounced in some than others).
Personally gone through the Warrenty process on an alloy 26er recently. They couldn't find one to replace it (being 26), so ended up with an SL0 frame instead (still 26).
I'm not sure about the stiff seatpost thing, but will say that after it cracked, I pulled out my newly installed carbon post (only been in about 4 months with little riding, and been using the giant alloy up to then), and there was extreme wear marks on the post from the back of the seattube near the clamp, and the front of the post at the very bottom, whihc will be evidence for someone on what's happening (can't be bothered to think that through at the moment)