#!/bin/csh
rm -rf ~/grab.jpg
screencapture -i ~/grab.jpg
open /Applications/Preview.app ~/grab.jpg
To make sure I could assign a shortcut to it, I used an AppleScript to execute this shell script -- and the AppleScript was then saved as an application:
do shell script "~bin/grabscreen"
Make sure you change the path and name for the script to reflect your script's location and anme. Now you can assign a shortcut to it using your favorite shortcut tool. This will create a screengrab (by selection; press the Space Bar for window mode), and open the resulting frame in Preview. I'd love to hear any alternatives to this hoop jumping...
[robg adds: I used Butler's ability to directly run AppleScripts and assign a keyboard shortcut to test this hint. In that context, at least, I found I had to use the full path to the file (/Users/robg/bin/grabscreen), not the shortcut to my home directory, in the AppleScript.]

