Blacksn0w NEVER jailbroke anything, it only unlocks.
Blackra1n is the jailbreak tool.
And you'd be rightto assume that the new blacksn0w is mainly for people who upgraded to 3.1.3 through PwnageTool or sn0wbreeze.
Apparently, the old blacksn0w had some issues with the CommCenter when running on 3.1.3, even if it was running the old baseband of 05.11.
PLUS, there were LOTS of Wi•Fi issues that people were experiencing on the 3G[s]. This release not only fixes the Wi•Fi issues, but allows people to PROPERLY unlock baseband 05.11 on 3.1.3 in a way where everything finally works perfectly.
It's more like an updated Blacksn0w version for "Pwn'd" or "breeze'd" 3.1.3 iPhones, designed specifically to fix unlock/Wi•Fi issues and to get everything in line, finally.