The Tirex tile rendering system can use several rendering backends for creating the tiles. The WMS backend doesn't really do any rendering, it just requests the map image from a WMS server.
The backend doesn't do re-projection, so you have to use it with a WMS server that supports the Google-like Pseudo-Mercator projection. The EPSG code for this was sometimes called 900913, for a short time it was 3785, but the current (May 2010) official identifier is EPSG:3857. The Tirex backend will always request an image the size of a metatile, split it up into tiles and re-assemble them into a metatile.
With Tirex and the WMS backend you can build a WMS-to-tiles converter similar to TileCache. But because of the queue handling, Tirex can make sure that your WMS server is never overloaded.
The WMS backend is contained in the Debian/Ubuntu package tirex-backend-wms.