I am not sure if this is new in 10.4 or not. If it isn't, I appologize, but I don't have a non-Tiger system to test it on. In the Finder, do a File -> Get Info on an item (application, image, whatever). Click on the small icon near the top (the icon pasting area) and hit Command-C to copy it. Now launch Preview and choose File -> New from Clipboard. The resulting new file will have all of the icon sizes and resolutions available for viewing/editing.
[robg adds: If you can test this on a 10.3 machine, please do so -- if it's 10.4 only, I'll edit the hint to reflect that fact.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050531223937842