I'm still wondering how I could have forgotten to come here earlier and post the link to the mail with my suggestion :-)
Here you are:
About your question, Ryan, WebKitGTK exposes that kind of events through the ATK API at least, through the "text-caret-moved" signal, from the AtkText interface . Other than that, not 100% sure though, I guess you could conect to the "move-cursor", emmitted by the WebView , to get the same information without using ATK.
Hope this helps a little bit as well :-)