Update #7c: For those 3GS owners with the 06.15 baseband (and only those owners!), version 0.9.8b7c allows you to restore to the stock 4.3.5 IPSW, then simply run redsn0w to jailbreak. (redsn0w has a built-in fixrecovery that will get you past the Error 1015 you’ll see when you try to restore to the stock 4.3.5 IPSW with a 06.15 baseband)
https://sites.google.com/a/iphone-dev.com/files/home/redsn0w_win_0.9.8b7c.zip?attredirects=0
Update #7c: For those 3GS owners with the 06.15 baseband (and only those owners!), version 0.9.8b7c allows you to restore to the stock 4.3.5 IPSW, then simply run redsn0w to jailbreak. (redsn0w has a built-in fixrecovery that will get you past the Error 1015 you’ll see when you try to restore to the stock 4.3.5 IPSW with a 06.15 baseband)