dsimagry, Mark wasn't asking about a Mid 2011 iMac, he was asking about a mid 2010 iMac with a 2011 MBP. That link is not applicable.
Mark, I have a similar setup (same iMac, 2012 MBP with thunderbolt) and have been working on it for a while. While the Thunderbolt ports are supposedly backwards compatible with the Mini DisplayPort interface, a bunch of reading (and an experimental purchase (and subsequent return) of a $70 Thunderbolt cable to try it out) indicates that you can't do that with a Thunderbolt cable but that a Mini DisplayPort <--> Mini DisplayPort cable should work.
As I'm in Canada, it's been a right pain to get one of those (less selection on our Amazon than yours) but I finally got one and only just tried it out this morning. It _DOES_ work, but there are some display refresh/jaggies issues on the iMac. I don't know if that's maybe because the MBP is running on battery at the moment & doesn't have enough power to consistently drive the larger display or whether there's something else wrong, but I'll be spending the remainder of my long weekend trying to figure that out. The issue is that any mouse movement on the screen causes bands of what looks like loss of frame sync 'static'.
I'll let you know here if what I eventually figure out. On the up side, this is by FAR the closest I've gotten so far.