that's not we he said at all... The Roku 2 will play mp4 and mkv if it is encoded correctly i.e. h.264 video and aac/ac3 audio. My Roku 2 will not even recognize an avi file, so OP don't bet that it will work.
AVI and MKV are not the same at all. AVI and MKV are NOT media formats themselves, they are simply packages for containing the separate audio and video components necessary for playback. An AVI may contain, for example, a DivX video stream and an MP3 audio stream - the AVI container is responsible for holding all the data necessary for keeping them in sync as they both play back. The MKV format adds some things to the containers that AVI doesn't have - support for containing the h.264/AVC video format (best format for HD video right now, which is why you see it in relation to Blu-Ray), support for chapters, support for subtitles (if you have a subbed AVI, it's probably baked into the video stream), and playback menus.
That said, your MKV files should play on a Roku 2.
The extension .mkv is used on Blu-ray only. It is actually a .avi file. I don't know why they used mkv instead of avi. It would have been more simple, but we don't seem to want to do things the simple way.