Doh! I just realized this thread "took off"... I think one of my take-aways from the OP was an error in "work flow," as it were. Specifically, people seem to be throwing in an additional step to dither down audio to 16-bit prior to the mp3 conversion... even SONAR's embedded mp3 conversion "leads" me to believe this is the case (i.e. you must select 16-bit in order to make an mp3???).
 
When I convert a file, I pass the 32-bit float from SONAR to Audition via the Utilities menu. The audio (at that point) is completely unaltered, then I do minor last-minute tweaking with Audition and use its converter to make the mp3.
 
So much behind that mp3 conversion is a mystery (to me), and Adobe certainly doesn't make it any more obvious. Since I use Audition's NR early in the process (and sometimes again prior to mastering), the noise floor is so "deep" that I do not get an audible difference in the mp3 created (unless driving the bit-rate down). Even 128Kb/s is pretty good, but I admit I have not yet pulled those mp3s back into SONAR to null test that. If I play the initial wave vs mp3 in WMP I do not hear any difference in these.
 
When I do think to do this test, I may actually do two passes... 1) pull the mp3 in with SONAR only and 2) have Audition convert the mp3 back to a 32-bit float first (i.e. undo its own handiwork).
 
Thank you for the replies! In reading this all, it leads me to believe that mp3s are not as "atrocious" as many try to portray. I will admit from a file sharing (and cell phone storage) perspective, they have been indispensable for me (I even batch converted a lot of mp3s down to 128 just so I could fit them into my phone).