Stupid (?) Network Question: Is ADSL Full Duplex?

[Kent Borg]

I have residential ADSL service from Covad.  I am not a regular bit
torrent user, but have used it a little recently.

Odd discovery: My download rate seems to be antagonistic with my upload
rate.  Using the bit torrent client "Transmission" under Ubuntu if I
leave the upload rate unlimited my download rate drops terribly.  If I
limit the upload rate to my download rate goes up!  Why?

I thought the bit torrent protocol rewarded those who supply bits by
letting them download bits faster, so if ADSL (asymmetric as it is) were
full duplex I should get my fastest download rate by not limiting my
upload rate, yet limiting to 15 Kbps seems to maximize my download rate.

Does this possibly have something to do with the NAT traversal stuff? 
(I haven't opened a reverse port for bit torrent.)  Is my Linux box
doing some stupid choking on these bits?  Is my DSL modem/router buggy? 
Is Covad doing something strange?  Have I hit some bug in the bit
torrent protocol or other implementations?  Is it a problem with doing
Wifi as my last link to my notebook?

Thanks,

-kb