If you rely on ultrasn0w unlock, this might come in handy. Just follow these simple instructions to find out the modem firmware / baseband version of your iPhone 3G / 3GS / 4. This way you can be sure if you phone can be unlocked with latest ultrasn0w or not.
Launch Settings from your Springboard.
Select General from the Settings Menu.
Select About from the General Settings Menu.
From the about screen you can find your firmware version under the Version label.
Scroll down and you can find your baseband version under the Modem Firmware label.