Unfortunately, you have been misinformed.
The spring update allows for Mpeg-4, while they are the same type of encoding, they quite different packaging.
The only way to play mkv is to use either transcode 360 (needs Vista or MCE), or Tversity. I have used Transcode360, but have never gotten mkv to work (video will stream, but will be discolored, out of sync, etc).
You could always re-encode using a program like Videora 360 or I use WME assistant (works great).
If you use WME Assistant, make sure you download Haali File splitter and Ac3filter, google for both. Once you have installed both of those, WME Assist should have no problem converting your mkv to wmv (keeps high def and 5.1, if you want).
And don't worry, those mkvs will be playing VERY SOON...some very smart people are bringing the 360 real close to being fully hacked...XB360MC will play anything you want! the xbmc team are already recruiting developers for the project...exciting!