http://wikee.iphwn.org/howto:random
"How do I re-enable internet tethering on 3.1.2 firmware?
A bsdiff patch to apply to CommCenter in 3.1.2 on the iPhone 3GS to re-enable tethering is available here.
(http://iphwn.org/CommCenter_3gs_312.zip)
It's just a 2-byte patch as shown below (and an appropriate readjusting of the mach-o's signature."