Yes, many of use with aftermarket rear sway bars (present company included) have run into this. Do a search in the suspension forum for swaybar fix, or 'ATI fixed my subframe' and you will see this is a common problem for those who tinker with aftermarket suspensions.
Yours would be the first mention I have heard of the stock sway bar leading to subframe problems.
I had the subframe pulled and some fabrication work done to solve the problem. You can find pictures of my fix
here . Here's a teaser:
[image]http://www.pbase.com/image/17985728[/image]
You might get away with doing a quick weld-in repair with the car on a hoist and the subframe still in there if you are lucky. I had to have the whole subframe pulled, which is actually pretty easy but prolly not something you want a muffler shop doing.