This worked for me and i now have a Galaxy Tab, on At&t making calls + 3G data + 850/1900Mhz WCDMA enabled.
**please note you need to have a SIM card with a proper voice plan and DATA plan for this to work. do not try this with the prepaid card that came with the Tab.
1) Download the firmware P1000XWJJ4 from HERE (pass: samfirmware.com)
2) download the firmware P1000XXJK5 from HERE (pass: samfirmware.com)
3) download Odin 1.3 from HERE
4) download the modem_bell_i9000.zip file attached to his post and extract it.
A) put you tab into download mode and flash P1000XWJJ4 with P1_add_hidden.pit and repartition selected. The phone will reboot and will be in Russian.
B) Put the phone into download mode again. flash P1000XXJK5 with NO repartition and NO PIT file. the phone will reboot with the latest euro firmware in english.
C) finish the setup wizard, if you do not get DATA at this point (3G or EDGE) at this point go into APN settings and change the APN from "Broadband" to "wap.cingular"
D) Put the Tab into download mode and flash the attached modem.bin file (the one you extracted) by selecting the bin file under "phone" in Odin. NO repartition and NO PIT file.
E) At this point your phone will reboot and you should have 850/1900Mhz enabled and everything you should work just like the euro version of the Tab.
Done and done.
UPDATE: I found that at&t are disabling HSUPA on the Tab, thus using the at&t original modem will not be ideal. I found that the Bell Galaxy S 2.2 modem is the only modem i found so far that will give us 850/1900Mhz + HSUPA + voice calls.
Please flash this modem instead. it is attached.