It was a 42t Shimano SG. When I inspected it after the break one of the chainring bolts was slightly loose, and granted, I'd say that would have been a factor. I feel quite confident that my new 1/8" ring would not snap if it had a slightly loose bolt though (*touch wood* But I'd hate to be proven wrong.)

edit: and also granted, there are much tougher 3/32" rings than the one I snapped. I'm also not sure if it was the spider on the Ultegra crank which snapped first, or the chainring.
I bought some Omniums which come with a 1/8" ring, so that's what I'm running now anyway.