Nope, no native support for mkvs. Maybe 3rd party players are mature enough to work with snapdragon hardware acceleration?
This is a good one... the official Thor trailer (It's a great 'real world' test, thanks to flash not using any video decode hardware yet and the cpu/media processing engine taking the strain

)
http://www.youtube.com/watch?v=JOddp-nlNvQ
Here's the current standing,
360p = OMAP3630@1Ghz+ (64bit NEON, VFPv3 not pipelined) 480p+ failed with occasional lag. Hummingbird phones too?
480p = Snapdragon@1GHz (128bit NEON, VFPv3 pipelined) 720p failed but it was v.close (slight audio lag etc)
720p = Tegra 2@1GHz+ (No NEON, VFPv3-D16 pipelined) and 45nm Snapdragons, DHD/Arc? etc.
1080p(?) = Does the SGS2 play it smoothly?
[email protected] (64bit NEON, VFPv3-D16 pipelined)
Either the sensation will have identical performance to the DHD/Arc @ 720p (using one core only) or it'll play 1080p amazingly well...