I have had this issue on multiple iphones I've had (3G on 3.1.2, and a different 3GS on 3.1.2), but for some reason when I turn on my wifi no networks are found. Since I've had this problem before I have researched it a ton in the past and I know its a common problem with 3.1.2, but the odd thing is it started working all of the sudden and all was great. Then on accident my phone died, and since it was a tethered jailbreak I had to rerun blackra1n, since then it is back to finding no networks, im going to keep rejailbreaking to see if itll come back, but im curious as to whether there has been a solution that has evaded me over the months, anyone?
if you have your shsh's for 3.1.2 then restore and use spirit to jailbreak, it will be an untethered jailbreak, I had no wifi issues using spirit to jailbreak my old 3GS on 3.1.2, it's worth a shot