Never had this problem before! When I bounce the tracks in a current project to final mix I end up with about 30 % difference between right and left channel (more to the right). Before bouncing, during playback, the balance is normal as also the meters on my VS 100 (soundcard) show. All I/O pan meters are centered on sends and busses. Tried bouncing a mono track (bass guitar) - same result. Picked up an old project and bounced a mono track there as well with no perverted R/L-balance. The problem is probably isolated to this last project then, which in my mind rules out any hard ware or OS errors.

Check if you have any gainstage on the hardware main buses. If you have a gainstage there and bounce one of the other buses down your mix will obviously be different. You can also try bouncing the mains themselves - the what you hear preset does this if I remember right.

Thanks Noel, appreciated!
The problem is I don't know what "gainstage" is... (can't see anything like that on my soundcard nor the midi controller).
I did unplug the Impulse 61 Controller to see if some settings there would have caused this problem but bouncing a mono track (bass guitar) ended up with the same L/R unbalance. Then I imported all the audio tracks from the "problem project" to a new one. Of course, I'll have to add all the plugins, set automation values and so forth, but now bouncing down the tracks kept the original L/R-balance. SO it's something within the project (soft ware issue) that causes L/R gain drift while bouncing.

Check your plug ins to make sure none of them are affecting panning.  To do this quicky you can disable all of them using the E hotkey.

i had a similar situation that turned out to be panning in breverb. i had panned the dry when i meant to pan the wet...

Nope, disabling fx:s including breverb changed nothing . Breverb has btw a very peculiar way of panning both dry AND wet with no measurements given! Hmm...

Deleted all busses and fx:s in every single track. Still this strange phenomenom...

Looks like this:
 

 
A bit blurry, sorry. The lowermost track is the final mix and as you may see the right channel is much thicker.

Cakewalk... Please help!!! I'm having the same issue. I don't remember having this problem before Platinum.
Urgent!!!!!!!
 
 
(UPDATE!!!!)
 
Hi Equality,
 
I found the solution. I tried to upload the image but I couldn't. Sorry.  It's like what they said, "the gainstage on the hardware main buses." Inside Sonar, you have to be in the mixer screen shot to see it. In my case screen shot 05.  Look at H/W out on the right most screen. In my case, the L/R sliders positions were slightly off. Hope that helps.