Good unit bit not perfect. Relies on a single SMS message to alert you.
http://www.edgarsson.co.uk/gsm.htm sell a GSM unit that is pretty much the dogs, will dial up to 4 different different numbers and send 4 SMS, which is very useful if you happen to have the mobile off or want it to call a land-line number.
You can also listen in and control up to 4 outputs remotely, use any PAYG SIM.
The downside is that you will have to package this for addition to a bike as it is primarily designed for static alarm systems, and can be purchased with or without the larger metal enclosure.
I thought that being just a little bigger than a cigarette packet at 105 x 76mm and about 15mm thick it would sit nicely inside the fusebox, and therefore it might not need an enclosure.
See also
Mobilarm or
Spy Equipment or
Phonesafe