Repetition #37: Not before the Mac Pro.
If we don't see BluperDrives in the Mac Pro (and consequently, ACDs with HDCP) at WWDC this year, then we won't see them. Meaning that the iMac will not be able to play Blu-ray movies natively, ever.
Here's the order of BluperDrive options if they are created:
1st: Mac Pro
2nd: MacBook Pro (possible simultaneous with #1) two months later
3rd: iMac 24" (simultaneous with MacBook Pro if it is not done with the Mac Pro)
4th: iMac 20" at the revision after the 24" gets the option
5th: MacBook
6th: Mac Mini... NOT! It will easily be dead by then.