In my book there are two methods to using profile links;
1. If your out to create lots of profiles in an automated fahion then you will want to create them on a high PR TLD. The power of the TLD will gradually filter down through the site to your profile. Worth noting this may not manifest itself in your profile page gaining PR. It is my belief that regardless of what your green bar shows, a profile link placed on a high PR and therefore trusted site, gives power.
2. Whenever you comment on a forum, generally speaking a DF link will be anchored to your comment back to your profile page. To take advantage of this, comment upon high PR posts thereby creating a PR juice flow back to your profile page. Do this regulary and within a few months your profile page will also accure some decent PR. This combined with a high level and trusted TLD will ensure the link is truly powerful.