% of US homes with a security system?

It's apparently a little better (or worse, depending on your viewpoint) than that. Parks Associates estimated that 18% of US homes had some for form of electronic security or home automation system installed as of 2001. Their projections, based on studies conducted over several years indicated at that time that by 2009 about 30% of US homes would be electronically protected.

Here's a link to their website where you can find more information on the subject. Not having seen their full report I couldn't tell you how accurate it might be but they are one of the more respected sources of industry data.

Of course the real question is how many people actually use there alarm system