Fastmac announced to have in its catalog a slot-in BD burner featuring a 12.5mm thickness, making it compatible with all iMacs (including PPC G5-based models) and MacBooks (except for MBPro 15"). Its price, US$1000, is not really cheap, but it will give you the ability to burn BD-R at 2x, support for BD-R double layer burning, DVD at 8x and CD at 8x.
To be noted, if you can burn BD media, you will not be able to play them as none of Apple hardware models feature the required HDCP chips; it would be about time for Cupertino to introduce such chips in all models.