Now, I consider myself a fairly smart person, but after seemingly endless research, I am still unsure as to what settings in PwnageTool will create an IPSW with the proper 02.28.00 baseband firmware. It's gotta be the "Enable Baseband Update" checkbox, which is greyed out for me, in the General settings, or maybe something in the Bootneuter settings.
One set of instructions for jailbreaking and unlocking from iClarified, tells you to download two bootloader files, but that is for a 2G, and I have a 3G.
I'm still not sure if it's worth it to upgrade my already JBed 3G from 2.2, but I like to know these things if I get a hankering.