....and the iphone jailbreak/unlock scene in general! Bought a used 3GS (16GB, old bootrom) the other day to use as a 2nd phone for a business venture and here's how it went 
:
1) Came with iOS 5.1.1 installed, never been previously jailbroken and locked to a local carrier (Rogers).
2) Knew I wanted to keep it on 5.1.1 so I jailbroke it using redsn0w, but then I realized it had a more recent baseband version than ultrasn0w 1.2.7 was compatible with and I had never dealt with baseband issues before because my iPhone 4 is factory unlocked.
3) Used redsn0w to upgrade my baseband to an ipad one that appearently breaks multiple functions on the 3GS but allows you to unlock it. THEN I found out redsn0w could downgrade me from that newer ipad baseband to an even older one than I originally had installed.
4) Downgraded my baseband, jailbroke my phone again, and installed ultrasn0w. Used redsn0w again + itunes restore to install a custom ipsw to preserve the older baseband on it.
Confirmed with another carriers SIM and right now, it is truely unlocked and jailbroken on 5.1.1! I am surprised how simple it all was, having only to use 1 program really, and it runs straight from the .exe. It is very well coded...The only issue I did run into was I bricked the thing trying to install the custom ipsw via iTunes on my Windows PC as it through out a different error each time and put my phone into a weird version of the "recovery mode". I couldn't get it to boot normally by re-jailbreaking it, or using the redsn0w recovery fix....Anyway, luckily I had my recently purchased MBP beside me and for whatever reason, I was able to restore the phone with the custom ipsw from OS X without errors and the phone is 100% fine now....
:1) Came with iOS 5.1.1 installed, never been previously jailbroken and locked to a local carrier (Rogers).
2) Knew I wanted to keep it on 5.1.1 so I jailbroke it using redsn0w, but then I realized it had a more recent baseband version than ultrasn0w 1.2.7 was compatible with and I had never dealt with baseband issues before because my iPhone 4 is factory unlocked.
3) Used redsn0w to upgrade my baseband to an ipad one that appearently breaks multiple functions on the 3GS but allows you to unlock it. THEN I found out redsn0w could downgrade me from that newer ipad baseband to an even older one than I originally had installed.
4) Downgraded my baseband, jailbroke my phone again, and installed ultrasn0w. Used redsn0w again + itunes restore to install a custom ipsw to preserve the older baseband on it.
Confirmed with another carriers SIM and right now, it is truely unlocked and jailbroken on 5.1.1!
I am surprised how simple it all was, having only to use 1 program really, and it runs straight from the .exe. It is very well coded...The only issue I did run into was I bricked the thing trying to install the custom ipsw via iTunes on my Windows PC as it through out a different error each time and put my phone into a weird version of the "recovery mode". I couldn't get it to boot normally by re-jailbreaking it, or using the redsn0w recovery fix....Anyway, luckily I had my recently purchased MBP beside me and for whatever reason, I was able to restore the phone with the custom ipsw from OS X without errors and the phone is 100% fine now....