I recently downloaded the newest version (0.8.5) of bitcoin-qt to my MacBook. I'd been using 0.8.0.
Today, I just happened to notice that my internet performance was sluggish so I took a look at the Activity Monitor and saw something was uploading to the internet at max speed. It was rolling along at 600+Kbytes/sec, which maxes out my Comcast cable upload capacity.

By process of elimination, I determined it was bitcoin-qt. Now I can understand it having to download the blockchain, but what on earth is it uploading?

Thanks