Aside: Does anyone here see value in an image resizing web service? I think it could be useful, particularly in environments where imaging libraries are in short supply (i.e. shared hosting).
Given the right set of features, yes [translation: "if it does everything I want" :) ].
Off the top of my head, if it worked like this gem (ability to request arbitrary sizes on the fly) and had the ability to do simple color transforms (B/W, Sepia) that would be cool.
If you could convert RAW files, that would be interesting as well.