I noticed it as well and I am not sure why it is ... I just took the main mixer and bumped it up to correct the problem ... I am still of the opinion that I will stick with 1.5 until 3.0 comes out ... WILLIE
Audition 2.0 implemented support for the Panning Law on the master output, which can be disabled in the Preferences under Multitrack->Stereo Panning Mode.
Panning Law, if you're not familiar, is the term used to describe the behavior of mixing consoles when a track was panned hard left or right, as compared to centered. The amplitude when panned hard was quieter than when centered, due to linear gain increase/decrease on each channel to simulate the pan. When centered, both channels played at full gain, giving an apparent +3dB gain as opposed to hard-panned to one side. There's an excellent explanation of this over at Harmony Central: http://www.harmony-central.com/articles/tips/panning_laws/
This shouldn't be an issue when creating new sessions, but you'll definitely notice it when importing sessions from 1.5. Changing that preference setting should prevent this from being an issue.