I have labeled markers working with the control. It took a few changes to the labeled marker javascript as well as the GoogleMap control source.
1. Download the javascript from: http://gmaps-utility-library.googlecode.com/svn/trunk/mapiconmaker/1.1/src/mapiconmaker.js
You'll have to modify all of the creation functions in there. Add the line "var id = opts.id;" at the beginning of each function and then after the GIcon is created add "icon.ID = id;"
2. In the GoogleMap control source you need to modify GoogleIcon.cs. Change the RenderString function definition from internal string RenderString(GoogleMap map) to public virtual string RenderString(GoogleMap map)
This is so you can override this function in your custom class.
3. Create your custom class. Mine isn't finished, but it is functional. My project is in VB.net, but it should be easy to convert it to C#. You can download it here: http://dl.dropbox.com/u/57205/LabeledGoogleIcon.vb
Then to use your new icons all you have to do is:
Dim icon1 As New LabeledGoogleIcon()
icon1.ID = "GoldIcon"
icon1.primaryColor = "#ECD58F"
If you have any questions let me know. Dynamic icons are definitely a cool feature :)