launch application "Mail"
tell application "Mail"
-- set parameters for subject, body and sender
set newMessage to make new outgoing message ¬
with properties ¬
{subject:"Your computer has Booted", content:"n/a", sender:"sender's email"}
tell newMessage
-- set Mail application to hide
set visible to false
-- set parameters for recipient
make new to recipient at end of to recipients with properties ¬
{address:"primary recipient's email address"}
-- set parameters for CC recipient
make new cc recipient at end of cc recipients with properties ¬
{address:"secondary recipient's email address"}
end tell
send newMessage
end tell
--quit Mail application
tell application "Mail" to quit
delay 10
quit
[robg adds: This older hint contains a shell script that can be setup via cron to do similar things, and it includes info like the output from traceroute. The problem with this AppleScript solution is that it will be triggered only when someone logs in to your account (assuming it's been placed in your login items list), not when the machine itself is booted -- and the odds are quite low that a thief will have your login password, too.
The earlier hint, since it uses a script that runs with cron, will run regardless of whether the user is logged in or not. That earlier hint also includes a link in the comments to this page, which is a very robust Perl script for doing all sorts of remote notifications.
Note that all of these protection mechanisms can be bypassed if the thief is simply smart enough to boot from an OS X Install CD, or another hard drive. Still, they provide at least a bit of defense for a stolen machine. Consider adding an open firmware password to make that task more difficult...]

