The complete list of compatible repositories can always be found in the Maemo Mapper Repositories thread, and now there's a new subsection for layered tiles, which I shall reproduce here for convenience:

Layers
These repositories contain transparency and are thus ideal for use as layers on top of the above repositories.

Google Traffic
http://mt.google.com/mapstt?zoom=%0d&x=%d&y=%d

Google Labels
http://mt.google.com/mt?v=w2t.999&x=%d&y=%d&zoom=%d
(use with Google Satellite or Google Terrain for labels for cities, roads, etc., i.e. "Hybrid" maps.)

U.S. Weather Overlay (WMS)
http://mesonet.agron.iastate.edu/cgi...ANSPARENT=TRUE