Yes, that can be done in the latest nightly builds (0.9.0-svn):
http://nightlies.videolan.org/
If you, when playing a video, go to:
Tools >> Extended settings...
And then go to:
Video Adjustments and Effects >> Some random name
You can enable the "Rotate" modifier.
However, you may not find that particulary useful for rotaing it exactly 90 degrees (perhaps a text box could be added for this?). If you instead go into:
Tools >> Preferences...
Then select "All" for show settings. Then go to:
Video >> Filters >> Rotate
And change the Angle in degrees. The press "Save".
Cheers, Arite.