http://www.apple.com/imac/specs.html
Looks like the specs for the new iMac do not include a blu-ray disc player. I suspect that it is too small/thin for this pricey player and they are trying to appeal more toward the pc crowd which, as most people know, are used to computers that cost less than the average Macintosh.
I do think that the next towers that come out will be likely to have Blu-ray disc burners, but until then, I guess we'll have to buy that third party drive.