<p>Speaking entirely in technology-free terms, what is the best way to make a mobile friendly site?  That is, I want to make a site that will work on a regular computer but also have mobile versions of the pages.  Should I rewrite each page?  The pages will probably have different functionality, so should I rewrite the backend code?  Should it be an effectively different site with the same database?</p>

I added a mobile presentation layer to an operational site about a year ago. Based on the architecture of the site (hopefully this isn't too technology dependent for you) I added a new set of JSPs to accommodate mobile browsers (sidenote: see
http://wurfl.sourceforge.net/
for a great way to build mobile pages independent of browser type). Additionally some of the back-end functionality was changed due to the limited functionality of most mobile browsers. So, in short, the integration wasn't as painful as one would expect.
Good luck!