just wanted to post a quick warning: some old (and maybe some new?) shimano SS freewheels are essentially not removable by design. i recently ordered two cheapo shimano BMX freewheels from Quality (a big bike supply company -- i work at a shop) and, rather surprisingly, they are both the irremovable kind.
an example of a two-notched, irremovable freewheel made by falcon:
image from park tool
a picture of my shimano freewheel is attached.
park has, as usual, awesomely-clear instructions on what to do if you need to remove one of these suckers:
http://www.parktool.com/repair/readhowto.asp?id=46
(that second to last picture is pretty sweet, i might add)
of course, i installed one without realizing that it was evil. i needed to remove it, and luckily i got it off without having to take it apart a la park's instructions, but i consider myself lucky.
frankly, i'm surprised that shimano makes these, or at least used to make them. i would expect it from falcon, but not shimano. sheldon might want to add something about this to his site.