I bought a credit on davinciteam telling it i had a z780i and managed to flash my tm506 with a z780 firmware. I don't know how, but it worked. The menu was in a different language when it first booted up but it immediately gave me the option to change it back to English. I did not have an data indicator at near the signal at first, so i though that the z780 had that disabled as well, but in the settings, it was defaulted to not use gprs/edge/3g for data. I had to manually tell the phone to use data. Then it appeared! I sent the settings to the phone via the t-mobile configurator (the k850i settings).
Google maps w/gps!
Fm Radio with RDS Support.
No telenav. I downloaded it and manually put it on the phone and it still did not work. You are forced to pay for wayfinder. I don't know if it's any better. Have not tried it yet. Can't be much worse.
No t-mobile address book sync. (mobical fixes that!)
No Instant Messaging. ( i use eqo.com anyway. much quicker than the default IM app on the tm506 and it uses data!)
There is no 3g signal in my area right now (Tampa, FL) So i can't check 3g data but if i get a 3g icon, i will be happy! You can manually select gsm and 3g or just gsm in the menu options.
Here are a few pics.