<p>I'm not asking about converting a LaTeX document to html. </p>

<p>What I'd like to be able to do is have some way to use LaTeX math commands in an html document, and have it appear correctly in a browser. This could be done server or client side.</p>

MediaWiki can do what you are looking for. It uses Texvc (
http://en.wikipedia.org/wiki/Texvc
) which "validates (AMS) LaTeX mathematical expressions and converts them to HTML, MathML, or PNG graphics." Sounds like what you are looking for.
Check out Wikipedia's article on how they handle math equations here:
http://en.wikipedia.org/wiki/Help:Formula
. They also have an extensive reference on LaTeX and pros/cons of the different rendering types (PNG/MathML/HTML).
MediaWiki uses a subset of TeX markup,
  including some extensions from LaTeX
  and AMS-LaTeX, for mathematical
  formulae. It generates either PNG
  images or simple HTML markup,
  depending on user preferences and the
  complexity of the expression. In the
  future, as more browsers are smarter,
  it will be able to generate enhanced
  HTML or even MathML in many cases.
  (See blahtex for information about
  current work on adding MathML
  support.)
More precisely, MediaWiki filters the
  markup through Texvc, which in turn
  passes the commands to TeX for the
  actual rendering. Thus, only a limited
  part of the full TeX language is
  supported; see below for details.
  ...
Pros of HTML
In-line HTML formulae always align properly with the rest of the HTML
  text.
The formula's background, font size and face match the rest of HTML
  contents and the appearance respects
  CSS and browser settings.
Pages using HTML will load faster.
Pros of TeX
TeX is semantically superior to HTML. In TeX, "x" means
  "mathematical variable x", whereas in
  HTML "x" could mean anything.
  Information has been irrevocably lost.
  This has multiple benefits:
TeX can be transformed into HTML, but not vice-versa. This means that on
  the server side we can always
  transform a formula, based on its
  complexity and location within the
  text, user preferences, type of
  browser, etc. Therefore, where
  possible, all the benefits of HTML can
  be retained, together with the
  benefits of TeX. It's true that the
  current situation is not ideal, but
  that's not a good reason to drop
  information/contents. It's more a
  reason to help improve the situation.
TeX can be converted to MathML for browsers which support it, thus
  keeping its semantics and allowing it
  to be rendered as a vector.
TeX has been specifically designed for typesetting formulae, so input is
  easier and more natural, and output is
  more aesthetically pleasing.
When writing in TeX, editors need not worry about browser support, since
  it is rendered into an image by the
  server. HTML formulae, on the other
  hand, can end up being rendered
  inconsistent of editor's intentions
  (or not at all), by some browsers or
  older versions of a browser.