the best reading 2 links I found were:
rec.guns FAQ: III.C.2.q.7. Are Sig Aluminum Frames Weak?
and
Sig rust & frame cracking? - TheFiringLine Forums
both appear to be in excess of 10ys old.. couldn't find much newer info on the subject using various searches.
I'd almost have to say it's a non issue / snipe hunt.
either it happend a few tiees and was statistically insignificant, and fixed.. or it was a percieved problem, with rare instances of it occuring.
i have bought a new gun, upon the first time fireing it, the hammer broke. I sent it back to the factory for free servicing... got it back 2 weeks later.. cost me nothing.. gun has worked flawlessly for the last 20ys.. was not a sig

I've since collected many models of guns from that manufacturer... with no other issues...
good gun.. had a defect that was fixed... end of story..
I'm betting the sig issue is of similar style..