Currently the Wiki breaks things into Administrators and Users. Administrators can lock and unlock all pages, and users cannot. With the additional extension hooks to the Wiki module that 1.6 added, what you are intimating could be done (add a multi-select next to the checkbox for member groups to which the lock applies, for example), but I have not seen such a Wiki extension released among the community yet. Though I get the feeling that you may mean “hidden” instead of “locked” by referring to it as “available to”? Which would still be possible, but is an entirely different matter.
Of course, Namespaces may be the easy answer for you out of the box, as Namespaces have their own Admin and User settings.
WYSIWYG: Phoebe posted a beta of YUI WYSIWYG with the wiki the other day, and I think Patrick has used Xinha.