Eric,
Actually, a proxy for commodities is already in the larger Couch Potato Portfolios in the form of an Energy fund or ETF. That's one of the reasons the portfolios suggested in 'Storm have done so well. It's also one of the reasons the larger Couch Potato portfolios have done so well.
Once the core positions have been filled--- domestic equities, international equities, TIPS, international bonds, and REITs--- the next block is energy. So the Six Ways from Sunday portfolio, with six pieces, is 1/6th energy or about 17 percent. Even as you build the portfolios out to 10 blocks, the commodity proxy piece is at least 10 percent. Similarly, our portfolios at AssetBuilder have a commodities position that ranges from 4 percent to 11 percent.
Here's a link to an earlier column on proxies for commodities:
http://assetbuilder.com/blogs/scott_burns/archive/2008/01/11/making-commodities-part-of-your-portfolio.aspx
Scott