I always wondered the same thing... so I just tried it. Yep, they do come off and go back on quite easily with no harm to the pen.
I used one of these rubber finger grips (not sure what they are called; see the picture), wrapped it around the nib with a squeeze on the top surface and pulled. The two flaps of metal on the underside of the nib have a snap fit to the feed so it takes a good tug. Try not to hold the sides of the nib because they need to flex open a bit to move past the snap feature on the feed.
Click to view attachmentYou can check with the Stan, Ron Dutcher, Russ Stutler and sakura_zeppelin to see if they have a spare nib. Other than that, I think it might be pretty tough to find one.
Thanks for the inspiration.
/Woody