I believe it started today with this blog entry at The Map Room, but suddenly there is a resurgence of interest in generating topographical maps in Google Earth. I’ve spoken about techniques for doing this in the past (here and here). However, let me share the easiest way to view US Topographical maps in Google Earth. This is a portion of a network link created by a company called 3D Solar which does USGS Topographical Overlays 
automatically. You just zoom in to an area of interest, wait for five seconds, and suddenly you have an overlay of a topo map. Just turn off the “USGSO topo maps” network link when you are finished using it. I recommend saving this one in your “My Places” for future use. For the full 3D Solar collection of network links (which also includes weather, NASA MODIS satellite images, and Landsat overlays as well), check out this GEC forum post.