Video in fullscreen without stretching

I am working on a project with video player in it using Video object that plays flv files or FMS streams. The user can switch to fullscreen browser mode. And here is my problem - most of the time the aspect ratio of the video is different from the screen ratio so it looks stretched. I want to put black spaces left and right or top and bottom (depending on the current video and screen ratios) so that it looks normal without stretching.
Thanks in advance.