After I updated to Ios4 and jailbroke my 3g
I had no APN area to make changes to.. I just spent
about 2 hours on the net hunting down a "fix" and
here it is
redsn0w 0.9.5 BETA [iPhone Dev Team]

worked great, did not have to reinstall any apps
hope it works for you

ps: for some reason on my Iphone I have 3 blank icons
in dark grey. Don't understand how they got there or
what they are for, and I can't find a way to delete them,
anyone have this problem
Best
Dave