> About mixed html/non html wiki pages. I'm not sure what the solution could
> be. Many wikis don't allow html at all.

Yep. coWiki for example does not allow HTML at all, but stores
Wiki markup internally as XML, so all coWiki pages allow
transformation of the Wiki content into HTML as well as other
formats. Because the coWiki XML markup is a subset of HTML, full
HTML is not possible (but other things become possible).

TikiWiki stores HTML as HTML, and Wiki markup as Wiki markup.
Both are fundamentally incompatible, and overlap in
functionality. That is, even if there is only markp in a Wiki
page that could be rendered using Wiki markup (say bold text),
but this markup is written as HTML (as <b/> tag), it cannot be
preserved by someone not having HTML permissions (because the
bold text is rendered as <b/> in the edit field and subsequently
filtered out).

Conclusion: There must be a property that is part of the page
and that property is "uses HTML". 

A person not having HTML permission cannot create pages using
HTML, and (that is new) edit pages that have the "uses HTML"
property. The latter is necessary because allowing them to edit
pages "using HTML" will destroy the markup in these pages.

Persons having HTML permission can create pag{img src="img/wiki_up/wikiwyg.switch.view.gif" }es using HTML tags
(but only pages actually using HTML get the "uses HTML"
property), and they can edit all pages, if these pages are not
otherwise protected.

Also, that particular incompatibility between HTML and non-HTML
users would be dimished if Tiki would do things similar to
coWiki, that is, store pages as a variant of XHTML and translate
output for edit. HTML users would then get all markup as tags,
always. Non-HTML users would get markup translated into Wiki
syntax, and would be prevented from editing pages that cannot
be translated down into Wiki syntax completely.