since Apple is still signing for 4.3.2, for now, all you have to do is download iOS 4.3.2 firmware..
iClarified - iPhone Firmware Files
then Hold down the Shift (Windows)/Alt (Mac) key and hit the restore button (in itunes) select the downloaded firmware and let the restore process begin (i think the host file needs to be edited, anyone? mine is, but it always is. might want to google about this first)
*i searched google regarding downgrading firmware when Apple is still signing the previous firmware and did not have any luck but i think they will stop soon*
Edit: it appears as long as Apple is still signing 4.3.2 you will not have to edit your host file but your time is limited
4.3.1 and beyond will require shsh blobs that have been saved via
TinyUmbrella (these are device specific)
also, i would suggest downloading Tinyumbrella and saving your 4.3.2 blobs asap, just to be safe. the following guide is for when and after shsh blobs are saved
Downgrade iOS 4.3.3 to iOS 4.3.2, 4.3.1, 4.3, 4.2.1 [iPhone 4, 3GS, iPad 2, iPad, iPod Touch] | iPhoneHeat
please search as much as possible before doing it