Why do you want Blu-ray on a 27" screen? Unless it's for data purposes, in which case, aren't peripheral drives supported?
Are you being intentionally obtuse? The difference between blu-ray and DVD is evident even on the 24" iMac display. You are typically sitting 18"-30" away from these things and the 27" will definitely start to show the lack of resolution of DVD not to mention the poor contrast and poor color space.
The rumors have previously indicated that Blu-ray was planned but was pulled at the 11th hour due to some issues.
I hope that's true and that Blu-ray arrives soon in a springtime refresh. I want that 27" i7 iMac but not without a Blu-ray drive.
Take a look at the reviews of Blu-ray movies like Braveheart and tell me you can't see the difference in picture quality on your monitor.