I use http://github.com/markevans/dragonfly for this purpose, using rack/cache to hold on to the generated images (also with no expiration necessary).

Right now I'm working on doing streaming intermediate resizes that never load the full decompressed image into memory, because I need to be able to take uploads of 50 megapixel images and be able to respond with new versions based on user input in a timely manner. RMagick may be an extremely shitty implementation, but even a more solid one is stymied by the algorithm.

The solution is to just do something else -- for JPEGs, libjpeg has the capability to do a cheap streaming resize to 1/2, 1/4, or 1/8 by partially sampling the 8x8 DCT blocks and for PNGs MediaWiki's image server includes a utility for doing streaming resizes, though it's finicky about the exact scale factors it'll allow: http://svn.wikimedia.org/viewvc/mediawiki/trunk/pngds/