First, the MC model thing doesn't apply to iPhones, only iPod Touches. I know it doesn't because my 3gs is an MC model and I have the old bootrom and jailbroken iOS 4.
Second, if I'm not mistaken the problem lies in the iPhone 3GS new bootrom. All the jailbreaks EXCEPT for Spirit are jailbreaks at the bootrom or boot loader. Apple introduced the new bootrom as an attempt to prevent jailbreaking. I think Spirit jailbroke the operating system after the boot, it's called a userland jailbreak. Spirit circumvents Apple's attempt at the new bootrom.*
@comex is working on a new userland jailbreak a la Spirit that should take care of the 3GS new bootrom.
*(I may have mis-spoke some of this but I'm fairly certain it's accurate enough to have the right idea.)