Just don't upgrade the device.. I'm sure there are so many iOs 5 users who really beg for having iOs 4 to be able to jb the device.. In that way, you're lucky.. As a second option instead of redsn0w; just write
JailbreakMe 3.0 (click to the url to see the link) to your iPhone's Safari and the rest will take care of itself

Than install ultrasn0w as it mentioned above..