Yep - "don't know what happened" was a bit overstated, I just wanted to put it simply. "Don't know quite what happens when you Sublime" would be more accurate. I think it might be Excession or even Phlebas that first brings up the idea that the Culture is unusual for reaching such a high level of development without subliming?
Yes - one of the later novels does mention that other advanced societies do regard the Culture as being a bit immature in their insistence that they won't sublime.
Incidentally, Subliming reminds me a bit of the transition from the Beyond to the Transcend in Vinge's Zones of Thought stories.
