It ought to be pretty straightfoward to write your own HTML to XHTML converter servlet, cgi bin page, etc. using something off-the-shelf like Tidy or JTidy? O'Reilly has something along those lines here: http://www.oreillynet.com/network/2000/04/28/feature/index.csp.
Off the top of my head, it doesn't sound like a great deal of work.
For an HTTP GET, it is probably relatively straightforward and perhaps
with some googling one could find lots of helper packages and APIs
for converting HTML to XHTML.
Theoretically, a MIDlet could also do a conversion to XHTML too.
But MIDlet size and memory (e.g. for large HTML pages) might be
Your mileage may vary :-).