If you don't want to go though the many hoops that I've described earlier to get USGS topo's as background in Potlatch or Merkaartor and you don't want to use the slow and sometimes inaccurate terraserver WMS service (which doesn't work with Potlatch anyway), there's another option:

There is at least one website with free online USGS topo maps with a Google Maps interface and tiles that are TMS compatible.

Just surf to this site, enable FireBug (on Firefox) or Web Inspector (on Safari) to analyze the resources and you'll be able to see the URL's for individual tiles.

Replace zoom/x/y.png by !/!/!.png in Potlatch as a custom background and, voila, fast and high quality USGS topo backgrounds.