Since Prowl is based largely on Growl <http://growl.info>, an icon similar to Growl’s is of course the goal. The current icon—a plain, boring cat, see <http://prowl.weks.net> —was something quick to get it out there.
One idea is something “catty”—the Prowl/Growl nature suggests it it, but I’m open to different takes on it. For example, Growl’s logo is a claw, though it’s quite outdated and ugly these days, but you can avoid cattyness altogether if you've got a good idea. Feel free to drop the cat idea entirely if you think you can pull it off.
An icon that shows off what Prowl does, instead of a cat nature (at least directly) are also acceptable and encouraged. No envelopes though, Prowl isn't an email service. ;)
iPhone logos have a mask auto-applied by the OS to generate the “gloss” and rounded-rectangle effect. This is an optional thing; we can disable it if it gets in the way of the design, but the rounded-rectangle is a requirement.
The icon needs to look good at 512x512 and 57x57. The former is displayed in the iTunes store, and the latter is displayed on the home screen. A 29x29 version should look recognizable, but clarity isn’t at all important, just recognizability. The 29x29 version is the one used in the ’Settings’ application for identifying the program.