I tired to find whitepapers to see if anything had changed in 4.0 vs 2.3. The LT28i and LT28at both only had 2.3 papers available and the LT28h had 4.0 available (since that's what it shipped with). There doesn't seem to be any difference in the papers other than the standard web browser/operating system upgrades, so as of right now I can't find anything that lays out any upgrades in file format support. Though even the LT28h didn't list support for .mkv, so I don't know. Although, I'm guessing you were using one of the web browsers to view the .mkv video? I know the support for files in the web browser seems to differ from that of the gallery viewers. For example .gifs will animate in the standard stock web browser, but in the Image gallery they only display the first frame of the file.
