hey guys thanks for all the help finally i've been able to do it.. all i had to do was to reset the phone before putting it into DFU mode.
so if any of you planning to upgrade ur iphone to os 5 just make sure you cook a custom build using sn0wbreeze in the 'preserve baseband mode' (this step is really important cuz if you upgrde with stock firmware baseband will automatically be upgraded to 4.11.08, and you won't be able to unlock it.)
, erase all content and reset the iphone (settings->general->reset..), put the device in the DFU mode and upgrade with itunes by hitting shift+restore using ur custom built firmware.