Over Robyn from pro exotics has been spreading the virtues of burrowing soil substrates, and has basically perfectly outlined how to achieve the best, strongest, burrowing substrate. I believe it's something like 50% topsoil, 25% sand, and 25% vermiculite. I'm definately going to look into it this summer, though I'm not quite sure I'm completely sold. Mainly just the mess factor and it seems like it might be a pain to keep clean. I would think for anything smaller than what proexotics uses (go to their website or ask Robyn for some pics or something), it might be hard to keep from bacterial build up in the soil without changing it out every month or two. If you have a large enclosure though, where the animal is not forced to walk over/sleep on/etc. the area that had been "eliminated" on, it seems like a nice, natural solution. I have relatively small enclosures for my reptiles right now, so I think I'll stick with the old "dry as a bone" (as robyn calls them) substrates until I can get bigger cages made, which will hopefully be this summer.
What kind of reptile(s) are you keeping anyways?
ryan