At 08:41 PM 12/16/2009, Rocket J Squirrel wrote...
>I'll second Richard's call for audible alarm.

Here's a schematic: http://www.flatsurface.com/pics/buzzer.jpg

Could be easily built on a piece of perf-board.

Radio Shack parts:
Buzzer 273-066 $5.49
http://www.radioshack.com/product/index.jsp?productId=2062399

2n2904 transistors 276-2016 $0.79
http://www.radioshack.com/product/index.jsp?productId=2062609

1n4148 diodes 276-1620 $2.59/50
http://www.radioshack.com/product/index.jsp?productId=2062587

3.3K ohm resistors 271-1328 $0.99/5
http://www.radioshack.com/product/index.jsp?productId=2062345

Perf board 276-148 $1.99/2
http://www.radioshack.com/product/index.jsp?productId=2104052