In all fairness, PIE apparently supports a kind of AJAX. If for example, you visit the IE Mobile Team blog, you'll find an entire post dedicated to what must be done on the server side in order for things to work properly on the client side. Somewhat indicating the hoops one most go through to make the Web usable on the Windows Mobile OS, there's even a poster in the comments area who goes by the handle "PieMagician." I laughed pretty hard when I saw that.
Bottom line: The mobile Web as far as I'm concerned is very much broken. The old crusty WAP is about the only way to reliably get things done but that's hardly the mobile multimedia utopia that "they" (the proverbial "they") keep promising us. Know of other sites that simply break on a mobile device? Please share them here (in the comments below) and moving forward, I'll factor them into my smartphone testing.