If you are hard-linking to these sites and they are on every page, then placing them in an iframe is not a bad idea as it will prevent you from having a lot of external links "draining" PR from your website. (Actually, it's just preventing you from maximizing the PR on your site). The more of pages these are on the better this idea becomes.
If these links are on only a few pages then it really doesn't matter. Those few external links are really inconsequential.