MC keeps announcing double duration and twice smaller bitrate for files encoded with MP3 VBR 64kb/s.
To make sure the bug isn't related to the encoder plug-in ("MP3 Encoder VBR" 2.0.57, LAME 3.93.?), the file was encoded using both the plug-in and the latest available LAME stand-alone binary (3.93.1). No differences whatsoever.
Windows XP Explorer (add column "Duration" in view) AND Windows Media Player 8 (yeap, MJ/MC is so great I never bothered to update to WMP 9 ;-) ) both report the accurate duration (though WMP 8 is totally wrong on the bitrate).
The problem remains whether playing a file imported in the library or playing in directly from the explorer ("Play with Media Center" shell extension), wihtout importing it in the library.
Note: I experienced the same kind of problem for some 'LAME --preset fast standard' encoded files in MC 9.0 and was able to fix the duration/bitrate values by updating the library from tags. The "updating the library from tags" trick no longer works in MC 9.1.274, at least for the above mentionned bug (didn't have any problem so far with 'LAME --preset fast standard' encoded files in MC 9.1)