SheldonBrown.com has a fairly interesting comparison of the 3 frame metals (Fe, Al, Ti) and how tubing diameter and thickness relates them to one another. I think its linked to in the steel vs carbon thread thats currently ongoing.

The resultant answer was what a lot of people have said. It doesnt matter as much what its made of as who makes it and which of the materials characteristics are you more/less willing to pay for.

Steel is proven and often repairable, but to be lightweight it has to be thin which makes it fragile and expensive, not to mention the risk of rust.
Al and Ti come out very similar in the article, with the exception that Ti is just a lighter more expensive version of Al. These two also have the issue of metal fatigue from repetitive strain.

I'm a great believer in the idea of Steel being the best all-round metal ever engineered by man, but my last bike purchase was Al for this reason:

I figure in 5 years I'll either need a better bike or be bored with cycling; in a garage, all frame materials gather dust equally well.