Terry and pdxman, there is no doubt at all that particle board will deteriorate from moisture. A rustproof metal covering would certainly be better, or even a totally metal workbench, but I built mine nearly 7 years ago right after I bought this place and needed them right now before I bought a lot of the tools I now have.
If you look closely at the picture of mine, you'll see a layer of cardboard on the entire surface that I didn't mention before, and then I also do most of my work on a pretty thick layer of newspapers on top of the cardboard in an attempt to prevent any moisture from water, oil, or grease from getting to the wood. Certainly not the best arrangement, but it's cheap and it's worked for 7 years./w3tcompact/icons/laugh.gif