Explaination

1. set theURL to do JavaScript "escape(document.URL)" in front document

This line just gets the encoded url

2. set tshell to "curl  http://www.glo.li/toolbarshort.php?url=" & theURL & " | textutil -stdin -stdout -convert txt "

use the shortner, pass the resulting webpage data to textutil to strip out html

3. set the clipboard to (do shell script tshell)

copy result to clipboard

how to

Create a new applescript file
Paste in script and save.
To run on double click save as application.

I use FastScripts which gives you a script menu and allows you to assign keyboard shortcuts. I use ⌃⌥⌘G control-alt-command-G this means I just hit the key combo and the glo.li url is on my clipboard ready for pasting.

Update 2011-03-10

I think something has changes as I now get 'copy'''' after the url, this sorts it out:

tell application "Safari"

set theURL to do JavaScript "escape(document.URL)" in front document

set tshell to "curl  http://www.glo.li/toolbarshort.php?url=" & theURL & " | textutil -stdin -stdout -convert txt "

set theglowurl to (do shell script tshell)

set endchar to (offset of return in theglowurl) - 1

set the clipboard to text 1 thru endchar of theglowurl

end tell