Just ran an MKV test in Roksbox, and it indeed failed to stream; I stand corrected!
Roku is mainly intended to be an internet streamer. If you need more liberal support for local streaming, you probably will want to purchase a different box. I personally don't expect to see codec or container support to change much in the near future, but that's just my opinion.
Roku XDS, AppleTV 3rd Gen
15mbps on TWC, DIR-615 Router (wifi-N to Roku)
Panasonic P42ST30 HDTV & BD-85 Blu-ray Player
Onkyo HT-S5400 7.1 system
Netflix, Amazon Prime, Hulu+, season passes, MLB.tv, IW Browser Roksbox