You can write it as a website using desktop notifications.
I've quickly hacked together a page using Mt. Gox... It's not beatiful code (I have virtually no JS experience, but it seems to work (haven't tested the 5% detection yet, as there hasn't been a 5% change in the last 1/2 hour...). Should be up once I have some webspace. (Only works on webkit based browsers, have only tested chromium.)
HTML5 version here:
http://teajam.co.uk/btc/btcnotifier.htmlTo use:
Press "Enable Notifications" before the first use.
Press "Press to start" to start.
(Has to be on webkit, haven't tested on anything but Chromium.)
It checks MtGox every 5 minutes, and updated you once the price has change by 5% or more.
I'll probably flesh it out a bit and put in a better UI later today.
//Edit: Just to make clear this notifier is entirely independent of the OPs. I just thought it would be interesting to see how hard an html5 version of it would be.