This is discussed at devteam blog comments in the highest rated post there by sherif..

***************
"good day fellow jailbreakers & unlockers

i hope those of u with 3g/3gs are enjoying their updated unlocked devices, all props to @MuscleNerd & co. for making this possible , hopefully the iphone 4 on 2.10.04 and 3.10.01 follows within a good time table.

on a very important side note, before anyone starts to complain about battery drainage issues after installing the new ultrasn0w on the 6.15.00 updated BB, the culprit here is not ultrasn0w, it's the hacktivation process (activating the phone by pwnage tool or redsn0w rather than using an official carrier sim to do so) that u mostly did while jailbreaking.

The 4.1 and 4.2.1 ipsws unlike their old ancestors, seem to keep looking for valid push certificates all the time, either thru your wifi, or thru your cellular data network (which u only have after installing ultrasn0w and hence u accuse it) , this can be seen clearly if u have a cellular working data plan for 3g or edge and u watched the settings > general > usage > cellular network data (sent & received are continuously increasing even without using any internet traffics on the phone) , this will also render the phone going hot even in stand by mode and also very rapid battery drainage occurs plus consuming ur data if you were not on an unlimited plan.

so to resolve this, u either:
A) try to activate the phone using the official sim if u have it (even if the sim itself is old and not working) , or borrow it from a friend if it's possible, by any means, try to have an access to one and use it
B) if A is not feasible, u can install SBsettings from featured cydia packages and turn wifi and data off when u r not using it (shuts down both 3g and edge) , till C is done
C) there is a very nice module being worked on by a trusted jailbreak app dev, namely @sbingner , which ll resolve that issue for those having it, the details of which shouldn't be disclosed until he finishes his awesome work, till then, either use A or B to resolve the issue if it exists for you

sorry for the long post and thanks for your attention"
********************


Option A seems the best if one has the ATT card but he's foggy on the details and doesn't get much clearer in the comments.

I had success doing exactly what he said but had to fill in the blanks. Here's what I did using a new boot 3Gs on 4.1 and 6.15.00 with a mac and tmobile.

-Insert old ATT sim card and reboot.

-Open itunes 10 and hold option key and click restore and choose the factory 4.1 IPSW I had saved.

-I got an error in itunes and didn't write down the number but most likely it was the "3G/3Gs not being able to be on 6.15.00" error. This left the phone stuck on the screen with the USB cable pointing to itunes.

-I tried new and old versions of tiny umbrella to kick it out of (what I thought was) recovery but it stayed on the usb>itunes screen so I quit tiny umbrella.

-Ran pwnage 4.1.3 with the fixed 3g bundle thingy and deselected "activate phone" and chose not to update baseband since it already is. Entered DFU mode in pwnage and quit pwnage.

-Launch itunes again and option>restore and loaded the new 4.1 pwnage just created.

-Loaded tmobile sim card when phone was restarting after itunes finished loading new 4.1, when phone came up in itunes got a warning about non compatible carrier. Quit itunes and disconnect.

-load ultrasnow from cydia, when it finished I got a screen on the phone that said 'phone activated'

-phone and data now working from Tmobile AND I did not have to enter the epc(t?).tmobile.com in the data field in settings.


just checked and my battery is stable and not draining and my push errors must be stopped as I have zero data transfered. I also have reconnected the activated phone into itunes and do not get the non compatible warning I did before.

Everything is working great. Thanks to the DevTeam and friends.


I just figured I'd post this here as the devteam comments are overflowing.