The PS3 supports MPEG-1, MPEG-2 (PS and TS), VC-1, AVCHD, AVI, DivX, XviD, MP4, DVD (.vob)
Nope, it doesn't support mkv, but you can easily convert it to one of the supported formats using a program known as 'mkv2vob'.
Also, Bluray movies aren't encoded in mkv. Rather, it's usually either MPEG-4 AVC or VC-1. The only reason mkv is really popular is because the pirates support it.