QuickTime HD and WMV HD audio channels are mixed up.
How did I get to that conclusion? Well... I noticed that when playing QuickTime High Definition (h.264) trailers and WMV HD videos... when played with VLC... people talked in the back left channel instead in the front center one!
At first I though "that's odd..." and checked to see if that happened in their respective players (QuickTime and Windows Media Player 10) and in both of them all the talking was in the front center speaker. I tried BSPlayer and Media Player Classic and both of them have the 5.1 channels possitioned correctly too.
After that I checked other DivX, Xvid, DVD, etc videos that had 5.1 on VLC (0.8.4a) and the channels were working correctly so... that means that VLC is not working correctly only with those two types of HD formats.
I hope this info was of use.
Oh, and I have the "Force Dolby Surround" in Auto.