Last year I wrote a post about using overlays as lables in GEarth. This was partly in response to the fact that GEarth will allow you to change the size of a label font but not the font type, spacing between letters (known as tracking).
I've just come across TypeBrewer from Axis maps who were partly responsible for the excellent GeoCommons Maker! (previous post) and ColorBrewer (previous post). It gives you background information on how to put text on maps and also lets you play around with variables to see how different fonts work on maps.
I shall be exploring the Axis maps site in the future I think, lots of interesting stuff there.