MKVs won't be able to be streamed, but you can play them directly from a USB drive.
Most exciting for fans of digital media will be MKV support. You'll be able to play MKV files from a USB drive. Roku has also improved playback of MP4 files. Streaming via Wi-Fi has also improved thanks to newly optimized code.
The other new feature that Roku is touting is improvements to gaming that offers support for more titles. It's a Roku 2 specific update that will add new games to the catalog and, Roku hopes, attract a few more developers to the platform.