Partial success!!!
I flashed my Samsung Captivate with cyanogenmod 7.2 RC 1 last night and ran some tests this morning. The car now picks up all of my SMS text messages and displays a little envelope icon on the screen when I get a new one. Haven't tried emails yet.
However, the car doesn't read them out loud automatically. You can select the message in iDrive and click the play icon to have a message read, but it doesn't do it by itself. I'm beginning to wonder if it actually does this automatically, or whether I'm a victim of a
slick advertisement.
WARNING: Flashing your phone with a custom ROM can carry big risks and in the worst case you can end up with a bricked phone if you're not careful. Your mileage may vary and all the usual cliches apply.
EDIT: The manual doesn't seem to say anything about reading out loud automatically. Only that you can select the message and have it read out loud.