A bike alarm most usually does not have a tracker and it won't notify you with texts if the bike is tampered with.
My bike was stolen with an alarm on it and a number of chains and other security items on it. They just picked it up, put it in a van and it was gone.
The trackers that offer such functionality are extremely expensive and have a monthly subscription to their service making it not affordable. I can't justify paying £700-800 (first year and subscription) and then another £200-300 a year for such service.
If the bike is gone... having it tracked down doesn't really make a difference since by the time you get to it, it's already been stripped down and pretty much a write-off.
That's why I thought I should head down this path and try to make one of my own.
Having said that... would a Leonardo board, a tilt sensor and a gsm shield would suffice?