DefaultButton v2.0 has been released. The code has been updated to work with ImageButtons (though only on Netscape 6+ and IE 5.5+). Thanks to Matt Pelletier at EastMedia for help figuring it out!
The reason Netscape 4 is not supported is that input tags with type=image are not recognized as form elements, thus there is no way to access them. If you need to use image buttons, use v2.0 of the code. But if you are going to use regular buttons exclusively, use v1.1 of the code instead since it supports a larger range of browsers.
Check out the DefaultButton workspace on GotDotNet to download both 1.1 and 2.0 versions of the DefaultButton code and samples.