I got a Xena disc lock alarm, the XR1 model. It locks to the disc & if the bike is rolled the alarm goes off. Sort of a loud, shrill cricket sound. Also ordered a Gorilla compact alarm as a secondary back up. It wires up straight to the battery, no splicing into the wiring harness as with the much pricier Scorpion alarm systems.
http://www.xenasecurity.ca/home/current/lock/http://www.tricktape.com/browseproducts/Gorilla-Compact-Cycle-alarm.HTMLI wouldn't recommend ordering anything from the tricktape.com website though. They're VERY slow. Ordered my alarm 3 weeks ago & it shipped yesterday. The Xena was about $70 & the Gorilla was about $75 + shipping. Gorilla doesn't have their own website but I've read some decent reviews of them on the web.