The only free way is through hacking the commcenter via command line. You don't get the wifi hotspot capability but bluetooth and USB tethering still work fine.
This guide should explain it:
http://xsellize.com/topic/74123-patch-for-commcenter-ios4-v401-on-3g3gs4g/
I've got it working perfectly on my iphone 4 under 4.01. Why pay when you can do it yourself for free?
Holy **** you're my hero. Major props for posting/reposting that.