I got rid of JB to upgrade to 4.0 on my 3GS. Hopefully JBing happens on the iPhone 4, so when I get mine I can buy MiWi.
I had my phone fixed to permit tethering but from what I've read, MyWi is totally the way to go. And the trial of it worked well, too. (I think that was MyWi?!) I rarely ever tethered, but when I needed to, it would be nice to have it just work.