I've noticed around the web a few blue-ray external drives for PC's popping up, I know an iMac wont display HD but would it be possible to play the blue-ray disc through such a device since DVD's will be getting phased out sooner or later.
Not at this time, no. Mac OS X doesn't support playback of Blu-Ray video disks, regardless of what drive is used. All you can use a Blu-Ray drive for at this time is data storage.
dave, i am considering an imac but am hung up on the fact it does not have a blu ray writer. you mention that an external blu ray drive could be used as a storage device, i am looking to offload data from a flash based camcorder to a blu ray writer, do you think i can still use the imac for this purpose?
If you just want to use the BR disk for storing a large amount of data, as you would with a CD or DVD but with more capacity, yes. At least there are companies selling external Blu-Ray drives - OWC, for one - for such a use; I haven't used any of them so I can't say of my own knowledge how well they work.