AlphaImageLoader weirdness

The AlphaImageLoader filter, which is used for rendering transparent PNGs in IE, seems to play some funny games with URLs. I'm running into a problem with a URL that contains a “+” sign. I'm using the JavaScript encodeURIComponent function, which converts the “+” into a %2B. However, for some reason when the HTTP request gets spit of the other end of the filter, it gets converted back into a + character (as shown by ieHttpHeaders), and ends up munged on the other end in ASP.NET. Other encoded characters, like spaces (%20) work fine. If I use a regular IMG tag instead of the filter, it works fine (except, of course, no transparency).

Weird.