OK I found the software and after installing I can with certainty say that you CAN still downgrade the modem firmware of iPhone 4 with iTunes 10.4 and 4.3.5 ipsw
I just bought off Craigslist for $250 a nice iPhone 4 with otterbox but I told the guy if I couldn't downgrade the baseband I wouldn't buy it. He had upgraded to ios5 which changed his baseband to 4.18.08 I think it was.
We met at Starbucks and I restored to 4.3.5 which Apple is still signing and now I can go research which gevey sim to use to unlock it for T-Mobile!
Oh and I tried it with 10.5 and it wouldn't budge or even let me restore from backup. I'll look at that when I get home again later.