after many different trys with tinnyumbrella and deleting the bootloader from the ipsw files, i found my own way...
when you load your restore file into itunes, itunes connect to
sourik server (if you set hosts file) or to apple server.
if blobs are saved or apple is signing your fw you whant to write to phone, then it starts flashing...
now you see on your iphone the progressbar.
now just disconnect your internet connection
because itunes whant to get baseband signature from apple server, it trys to connect to the apple certifacte server, but since you disabled the internet access, itunes whont get the certificate and therefore it cant update your baseband and will quit with error code... THATS GOOD!
now kick your iphone out from recovery mode with tinny umbrella or iRep or whantever else you prefer to use...
your iphone is now starting with new fimrware and old baseband!
ATTENTION: i have tested it with 3G and 3GS
3G is NOT WORKING, it will update the baseband!
3GS is 100% working with my methode!
4G i dont know... dont have tested it.
so for 3GS this is the saves prozess to make update without updating your baseband....
DONT USE THIS METHODE WITH NORMAL iPHONE 3G!
i think with iphone 4 it should work too..
wbr.
hari