I don't know if anyone else is having this experience, but the only time my MBP recognizes the TC via ethernet is when you pick the third networking option (about being connected via ethernet and wireless being disabled). That seems flipping useless to me, but the Genius Bar wasn't able to do anything better for me today. We even exchanged the TC for a new one (both ran 7.3.1 firmware), and nothing. If any other option's selected, I'll get HW-growler messages that the ethernet is connected but the TC won't show up anywhere.
I've also discovered that if you use the TC to connect to another wireless network (but not as the primary), that if your wireless connection to that device drops, you TC will also drop during a backup. Particularly annoying, since I have to run my Linksys router in B/G only mode for my MBP (due to a difference in 802.11n specs between Apple and other vendors, according to other threads on here).
Lastly, those of you that have connections that keep dropping during backups, beware of the SparseDisk file size. I noticed that despite having no available backups (due to zero successes prior to figuring out the above), the image on the TC kept increasing by the amount that the backup managed to get through. Apple didn't have a response for that one either, other than wiping it and starting over.
-ilija
Message was edited by: istojic