More information can be found on the Austin group's website: HSMM-Mesh.org including a link to a buying guide for WRT54G hardware. Other hardware may be supported, but this is currently the best list.
By registering with the site, you gain access to a few more links to information as well as updates when they happen. Also, check out the Yahoo Group: DAWG-HSMM which is made up, mostly, of mesh-net hams in the Dallas area.