I hope this is help: I Have also 3gs and stupidly updated to iPad baseband, though I understand the consequences that it will break the iPhone cellular, and it also happen exactly like what happen to your iPhone.
Then I did re-jailbreak again reflashing the baseband with Redsnow 0.9..9b7:
warning : ( firmware 5.0(9A334)) not '' 5.0.1''
by clicking both installing cydia & iPad baseband at the sametime. as it is taking a little bit longer and might showed up an error in the process you don't touch and even cancel the process.
wait till it finished and you see the apple logo appear.

P.S: as the baseband is already upgraded you can not ever use iTunes to restore, use Rdesnow to downgrade firmware if you want it to 4.3.3 untethered, but if you wanna keep it 5.0.1 also works fine unless it is tethered , but BIGBOSS' semi tethered is awesome, same state like untethered only when it really off you may need to booth tethered from redsnow 0.9.9b6---b8 worked fine to downgrade to 4.3.3 too
then again do only the boot tethered.

after finished you might need to install the BIGBOSS semi tethered source.
you will have your iPhone semi-untethered and unlocked, I can proved it really worked.

P.S: I did the mistake too removing the cables during the process as i was worried last time. but not too worried i rejailbreak again from the beginning by DFU mode on my device first and smoothly done.
note: patient & do again and again. work with mine so hopefully work for you too.
good luck

have you install BIGBOSS' source semitether? after installing not to forget to install the semitether tweak and restart! try it'll help.

try download ipsw from some other sites like felixbruns dot de or iClarified, it'll help!! happen to me too even host pointed to cydia.

I did as you suggested and works, thanks a million M!

Thank you so much, I clearly understand it. I'll have a try. Thnaks

Thanks M, is it unlocked too?last time i tried it was so bad, can't use my phone even after re-boot tethered.
how did you managed to do that? redsnow 09.6. b8?
really thanks again

Anyone on jailbroken iphone 3gs iOS 5 baseband 06.15.00 managed with BIGBOSS' semi untethered, is it work on your device? Please advice! Thank you so much

download ipsw 4.2.1 fhttp://www.felixbruns.de/iPod/firmware/ and http://greenpois0n.com/ and not to forget google

I suggest stay on 4.3.3 if you want it unlocked on baseband 06.15.00 fully functional, mine also ....giving up error... too just leave it till it really done apple logo show up and re jailbreak again by installing cydia and check iPad baseband again to re flash as I did on my 3gs and work & unlocked. on iOs 5 just wait till untethered out but may be unlock might not be available on iOS 5, even if on iPad baseband. I gave up using iOs 5 as it was not stable on my 3gs 06.15.00.
Good luck

remember keep on trying Redsn0w 0.9.9 b 6 / 0.9.9 b7 works fine with my 3gs last time, thanks to the generous Dev-Team

dude, i did upgraded my 3gs w/ iPad baseband, worked very well (except can't downgrade it to lower baseband w/ fuzzy band ) that the only risk. first was very tough to work on it, coz iTunes reject it, tiny umbrella don't help .The only way is use REDSNOW 0.9.09.b6 keep doing the reflash iPad baseband whenever it stuck in the pineapple, try and keep trying ( sometimes check install cydia also update iPad BB at the sametime, just do not disturbed during the jailbreak process, even if it said error, just leave it alone till it done. if it doesn't work, do it again just to re Flash baseband.
P.S: mine is on 4.3.3 I tried 5 too it work, except I hate tethered

Good luck
coz it really work with me; (edit-Put Twitter info in profile)