I tested the old "truth" that having the optin-box above the fold is pretty much essential (this was in the IM niche, so attention span is short).
test 1: optin box above the fold (to right of welcome video) AND at the bottom of 1.5 page letter.
test 2: same as above, except the optin box next to the video was removed (i.e. they had to scroll all the way to the bottom).
test 1 had 50% lower conversion rates than test 2.
ah well, you never know (and I only tested the 'above fold' idea because a well-known marketer told me to do it because I'd be killing conversions by not having it above the fold;-)