I know modules like Mobile Tools can do what I want - but they don't work with Pressflow or Varnish.
So, what I'm hoping to do is assign the sitewide theme based on the url used to access it. For instance:
example.com <-- This gets the standard, desktop theme
m.example.com <-- This gets the cool shiny iphone-optimized theme with flashy javascript etc
I dont' want multisite because the content is the same.
I don't want a new entry in domain access because the content is already written and it would (presumably) be a huge hassle to go back and assign it all to the new domain.
There's a Domain Context plugin for context, but it says it loads too late in the flow to be able to change the theme.