I have an iphone 3g, ios 4.0.1 and firmware 05.13.04
Recently i did jailbreak using browser based tool called "jailbreakme", then i disabled 3g and unlocked it using ultrasn0w 1.2. The problem is, my iphone is not detecting my T-mobile sim. It says no sim, no matter how often i put it in and out of tray. I read AT&T uses microsim for iphone, is that the case troubling my iphone? or any other possibilities?  any solution would be appreciated.

dude , try network resetting if not then hard reboot and reboot twice or thrice.if not  uninstall and install ultrasnow 1.2 again . reboot again wait for about 2-3 minutes.

 Nop dude it didn't workout. I tried every other method but it f**** me hard

I am not sure which sim card you are using with T mobile but you have pointed out the problem already. Iphone uses "micro" sim card, not the regular sim card. I had Sony Erriscon (from At&T) set and upgraded to Iphone 4 last year. I remember I could not use the old sim card and had to copy the contacts from old sim card to micro sim card for the Iphone.

Lapid,
Why don't you use redsn0w to jailbreak.. it's pretty easy. And no trouble so far.. 
if you need help, inbox me.. 

guchcha chor!

My bad...I have Iphone 4 which uses micro sim. But it looks like earlier version of Iphone (3G and 3GS) use normal sim card (also known as mini sim card) as mentioned in this forum. "Specifically, the original iPhone, iPhone 3G and iPhone 3GS use a Mini SIM, more typically called just a SIM card, and the iPhone 4 (GSM) uses a Micro SIM. " (http://www.everyipod.com/iphone-faq/iphone-sim-card-troubleshooting-upgrade-from-original-to-iphone-3g.html)

I think the best way to fix it is to redo again.if the base band is 6.5 and ios is 4 and above, jailbreak it using snowbridge which will unlock it as well and install ultrasn0w and yellowsn0w both. but still you have to reboot plenty of time. good luck

How to unlock 4.2.1 iphone 3gs?

 bro...download the latest redsnow...n download the ipad baseband...it seems like u didn't upgrade your baseband....and jailbreakme was patched long time ago....when u do the jailbreak select install the cydia and install the ipad baseband ...it will show u a dialog box saying u will not be able to downgrade frm the current firmware..selecet yes... and proceed with the jaibreak...then download the ultrasnoww....frm cydia....then that should work....i did it same way....
let me know if u need some help...

 Thank u all for yr input
i used 'jailbreakme'. now i have cydia and i installed ultrasn0w to unlock but still says no sim. may b the error is within the sim tray, i couldnt figure it out.
I could use redsn0w but the problem is i couldnt enter to the DFU mode 'cause power button is stuck. now attempting to fix the power button, hopefully it works.
by the way is it possible to enter DFU mode using program called 'sbsettings'  ?? it has option to power, respring,reboot, safe mode but i haven't seen DFU so wondering about it.