When dragging a form element onto a long form, the page doesn't scroll. These means you have to drop the element higher up the page, manually scroll down, then drag it again to the bottom of the form.
Which version are you using ? This has been adressed in R2.
The XForms editor have been overhauled in the R2 release and you now have a separate scroll for the form, so that you can scroll to the right place first and then drag a control into your form.
Perhaps I should have refreshed my browser before posting :]
I'm currently using SP3. Ok, thanks, I will upgrade.