Just to make it clear, iPhone Baseband Downgrade

Lots & lots of people and especially iPhone 4 users were asking how to downgrade iPhone baseband.So, it's the answer for them

In which conditions we can downgrade Baseband???

1. If your iPhone is running Beta ios and apple still supports previous ios. [Example:- Currently if you are running iPhone with ios 5.0beta 1 you can downgrade to ios 4.3.3, coz apple still signs firmware 4.3.3.As you downgrade firmware your baseband will also be downgraded from 4.11.04(in ios 5.0beta1) to 4.11.00(in ios 4.3.3).]
2. Tss server or the Tiny Umbrella Server just can only downgrade  firmware not the baseband, so if you can restore your iPhone to Original Apple official firmware without using any 3rd party utility or jailbreaking utility without getting a error, than you successfully downgraded iPhone baseband. 

Else the condition to downgrade baseband is, if you are a genious in using commands and can erase and install baseband getting apple to sign it without using Itunes.
Notice that:- Always restoring iPhone 3g/3gs/4g via Tss server to previous firmware gets error while verifying firmware means while erasing and installing baseband, coz none server else than Apple's server can sign baseband.