Use at your own risk. It crashes GE Plus repeatedly, even when used according to instructions. Needs work.

The topo maps are loading for me quite well, no problems here in GE beta 4 (I do get an error message, but just OK out of it and the maps load).

This crashed GE Free for me, but I found another link that works quite well: http://ge.gbif.net/gbifwmslinks.php

Has topos for Canada, US, and New Zealand. Give it a shot if this method doesn't work for you.

I don't understand why the GE team doesn't take this uncopyrighted "free" data and make it a Layer? Now in ArcGIS software...
http://www.spatiallyadjusted.com/2007/04/25/seamless-usgs-topo-in-arcgis/

I agree that it would be HUGE if GE would just make the USGS topo maps (7.5 minute series) a layer. What's the difference between that and a photo image? This is all open-source/public data... someone just has to scan it all in. In fact, if Google would just buy somebody like topozone.com, they'd have it done already.

I wish to propose the Google disegners to introduce a new layer set, that I call The History/Ecology Layer.
The Idea is to add some features that show Earth political borders and other features over the years. Besides the possibility to see how the political borders changed, I am looking for Historical features linked to human life in general: some topics I am proposing: the spread if Black Death in Europe (1347-1351), the Great discoveries, and so on.
And the ecological changes too, wich might be more important for educational purposes of the young generations: How the rain forest extention changed over the years, the spread of deserts, etc.
I foresee also a Time-Google Earth showing the future extrapolation of the polar cap melting and so on.

The idea is skeched in the following chapter, I wrote (in bad English) in my ignored webpage, even before Google Earth was on the web. TIME (Tridimensional Interactive Model of Earth), in fact, may be the next release of Google Earth.

see: "A letter to grand-grand-pa" at URL:

http://www.geocities.com/tgcp2084/5understand.html#letter_to_grandpa

I use the 3D Solar topo link all the time. In fact, I've got 3 of them so that I can get 3 maps displayed at once. There's a trick to it:
1. Save off the placemark from 3D Solar, giving each a different name on disk.
2. edit each one to change the internal name to something unique for each
3. Bring the edited placemarks back into GE. With different names, they won't try to replace the existing placemark.
4. In the properties for each, change the View-Based Refresh, under the Refresh tab, from "after camera stops" to "on request.

Now, when I want to get a topo map, I position myself in GE, check one of my topo placemarks, right-click/Refresh, and I get a map. I get as close to the ground as I can, around 2000ft up, and I'll get a failed load 'cause I'm too close to the ground. I then back out until I get a successful load. That gives me the most detail possible. I can now pan and zoom without losing the topo map.