You could find these on Google yourself, of course, but to save you the trouble;
Spokesman bicycles - Which Frame Material is Best?
(See also the sidebar, with the "Metallurgy for Cyclists" links).
Roadbike Review - Why You Should Be Riding Steel
Smartcycles: How Light do You Need Your Bike to Be?
IBike.org: Bike Frame Buying
Breton Bikes: "Steel Yourself"
Bike Race Info: Bicycle Frame Materials: Steel, Aluminium, Ti or Carbon
Funniest Thing I've Heard All Week
12 hours ago