One feature of MS Excel that is invaluable for business users is that you can not only password protect the document (for opening) but you can also force it to open as read-only unless the password is entered. (Password required to edit).
This feature has not been picked up by any of the other Office suites and I think it would help Linux in general and KOffice in particular move into the Corporate environment if this feature was added. This may then allow further corporate-funded investment and effort into KDE projects.

What about Tools -> Protect sheet?
(I'm translating from Spanish)
At least on 2.1.1 is there.
OpenOffice.org Calc have it also.
No idea about gnumeric, though.

Sorry I should have clarified - Password to edit on Opening. The problem with protect sheet is that after unprotecting it for editing, it then needs to be re-protected to make it read-only. This re-protecting generally gets overlooked and forgotten by the user.
Passwording to edit on Opening is basically automatic once is it setup, and is much easier for basic users to understand.