I have recently downgraded from iPhone 3.1 beta 2 firmware back to 3.0 with the guide provided as in my earlier post here.
When Apple announced the release of 3.01, I downloaded but not installing, fear that I will lost my tethering using Celcom 3G.
For your information, Maxis is the authorised Apple Carrier in Malaysia, and my carrier, Celcom is not.
What happened was in 3.1 beta 2 firmware, Apple imposed key signing to IPCC (Carrier bundle definition) which effectively stop unauthorised modification to the IPCC. Since Celcom is not an authorised carrier, there's no IPCC made for Celcom. I have customised one, managed to install, but failed.
That's the reason why I fallback to 3.0.
After fallback to 3.0, my baseband firmware remains 05.10.01, which is essentially the 3.1 beta 2 baseband version.
Thus, to update to 3.01, I have a slightly different scenario than most others who just update from 3.0 to 3.01, as their baseband firmware is 04.26.08.
So I really dunno whether update will screw up my iPhone 3G. I decided to give it a try, as a return of favour to @SpaceFlightO who has been generously lending his helping hand during my downgrade process.
@SpaceFlightO is having the same situation like me, and he is yet to test if his method still work for 3.1 Beta 2 -> 3.0 -> 3.01 where baseband is the latest version.
I did an update to 3.01, after all the waiting, iTunes complaints that my iPhone 3G turned to recovery mode, that I will need to do a complete restore.
I did a restore, then use the method @SpaceFlightO written in his post here, and iPhone 3G comes back.
After verifying with Apple on activation, my iPhone 3G comes back.
Instead of copying content from the backup set, I treat my iPhone as a new phone and sync all info back again.
What I lost are usernames and passwords that I have set for applications installed on my iPhone, means I will have to manually set again. Also SMS content is gone (of which I have done a backup via iTunes before I did the firmware update/restore, I still have a copy on my MacBook).
In short, I managed to update to 3.01 with baseband firmware from 3.1 beta 2, and I got my tethering back for my non Apple carrier - CELCOM.
Updated to 3.01, with baseband 05.10.01, using default carrier bundle which disables MMS and tethering by default.