If its on baseband 6.15.00 then theres nothing you can do. (Settings -> general -> about -> modem firmware)
Musclenerd said he would work on it this time last year but nothing came of it. If your using an iPhone 3G then i think you can use fuzzyband on cydia to fix it as far as i know. no solution for 3GS though
Thank you for the update. Unfourtunately I am using a 3gs. Its killing me on my motorcycle rides not having gps.