ShipIt!, currently on sale for $0.99 at the Mac App Store, offers a simple utility for resizing multiresolution images. Its suite of standardized image formats (such as 512x512 iTunes artwork, 57x57 iPhone icons, and 72x72 iPad ones) ensure that you can create consistent elements from your core art.
Drag your art onto the app window, choose a destination and the formats you wish to export to and the app creates the resized elements.
As utilities for a buck go, ShipIt may save you a bit of time if it fits into your development flow.
I found its user interface to be a bit crude, with the step-by-step process at the bottom of the window not really working for me.
I would have preferred to see more state information onscreen. When you pick the default output folder in "step 1", that directory is not visible anywhere onscreen.
I also wish the export file types were integrated into the main window instead of floating. As the floating window is, it cannot be resized. This means you can't just have all the elements visible on-screen at once.
What's more, the checkboxes didn't act in a standard OS X way -- they sometimes resisted attempts to uncheck them if they were the sole item selected.
In the end, ShipIt! seems to have the right idea but could use work on how it gets you to that place.