One important thing I would like to propose here for the developer is a "standard" location somewhere on the SD card which would keep all the common data such as contacts, recent sms, call logs, etc. so that multi boot environment can share data without loss or duplicates.
In other words, boot to CM6 tonight and in two days when you decide to go back to stock ROM, all your sms, phone calls and such are picked up right where you left them in CM6 and vice versa.
Other than that, this sounds like an awesome thing. Can't wait to have the phone boot one ROM for Monday, another for Tuesday, and another for Wednesday and another...