I recently did a round of testing in a whole bunch of browsers, and while I'm not too concerned about some of the really old ones, I did find some interesting problems with WebTV/MSN TV ( http://developer.msntv.com/TOOLS/webtvvwr.asp and http://developer.msntv.com/Tools/msntvvwr.asp ).
It seems that WebTV does its own conversion of quotation marks and apostrophes, but it does not recognize any of the numeric entities for Unicode characters. It just displays a question mark.
And yes, I have seen WebTV hits in my server logs.
I can probably hack something up that will check the user-agent string and call a custom function that skips texturize, but I wanted to bring up the idea of a more generic mechanism. Basically I'd like to be able to keep a short list of "low-end" browsers in the config file and selectively disable features that don't degrade gracefully.