I finally have a resolution to the n810 to MotoRIZR Z3 to Tmobile issue. I'm using the Tmobile Total Internet plan (adds $19.95 to your current voice plan).

The key, for my situation, seemed to be in rebooting the phone itself. After several frustrating days of messing around with everything, I deleted the pairing on both the Z3 and n810, then re-paired them.

Next, I turned OFF the phone, removed the battery and let it sit for a few minutes. After replacing the battery, and turning the phone back on, everything worked!

Here's the connection that worked:

Connection Name: T-Mobile Internet
GPRS

NEXT

Access Point Name: internet2.voicestream.com
Dial-up number: *99#
User name and Password are left blank

NEXT

Advanced

Proxies tab
Proxies: off

IP Addresses tab: Auto-retrieve IP address: checked on
Auto-retrieve DNS: checked on

Other tab:
Allow plain text login: no check (not sure if this has any effect)
Use PPP compression: CHECKED (not sure if this has any effect)
Supplementary AT commands: at+cgdcont=1,"IP","internet2.voicestream.com"


I was also able to speak to 'Zak' at Tmobile who actually had the "AT" script and setting info (as above). Nothing worked. We were, both rather frustrated, when we hung up. Then, I thought..."here's something I haven't tried, yet" and did the MotoRIZER battery removal, which rebooted the phone, and it all came alive!

This is now working.

Thanx to everyone who responded.
AndyR