Over the last year it seems bike theft is on a rise. Last year we had very fw reports of bikes being nicked; this week alone I've had 5. The best way to prevent this is, obviously, with a good lock. Notice i said a good lock.
We can argue what a good lock is for ever. Every lock can be defeated by someone with the know how. Locks are a deterrent, not a absolute stop.
That said, we carry some of the toughest, most secure locks on the market. from simple cable locks, if you're just running into a corner store, or the tough NEW YORK CITY lock from kryptonite with a $1500 guarantee. 


http://en.wikipedia.org/wiki/Bicycle_lock
http://www.jimlangley.net/crank/howtolock.html
http://www.chicagobikes.org/public/theftprevention.php
http://www.sfbike.org/?theft_locking