Here are some additional thoughts from an RF electronics background:
1. Any wireless alarm's RF signals CAN be jammed/blocked by a stronger signal on the same frequency. Any company that claims otherwise is lying, plain and simple.
2. Using a common X-10 transmitter to block the security devices is a crap-shoot. The odds are that a signal from a sensor will make it through at some time anyway. Don't just do a quick walk-test to check it; try moving around your home for 10 - 20 minutes searching for valuables and moving items outside, and if your system still doesn't go off, then go to #5 below...
3. The odds of someone building and using a powerful, boosted 310MHz jamming signal are slim to none. Especially if you don't advertise what type of system you have. These are the guys we see on TV running right into the arms of the cops, holding home videos of themselves committing their crimes, remember?
4. Code-grabbing is NOT as easy as some would have you believe. Unless you're in a very tight neighborhood, AND your neighbor is the one trying to steal your codes, it would be easier for someone to grab you on the way out the door & force you back into the house for a nice old-fashioned beating & robbing than to steal your codes. Don't be paranoid, the gang from "Mission Impossible" is NOT after your X-Box!
5. If you're really worried about jamming, you can tip the odds in your favor by keeping your signals as strong as possible. That means changing the batteries
before they get weak, aligning the transmitting units for best reception at the controller, and placing the controller in a central location so that your sensors are closer to it than any thief's remote, thus probably having a stronger signal.
6. Use plenty of hardened signaling devices outside. That means not easily found, nor easily removed. You want attention called to the place when the alarm goes off, in case you can't get the phone call for any reason. And don't be afraid to put 911 on the list of numbers; at least in MY small town, they WANT to know when you might be having a problem. I suppose in large cities they'd rather have you go home and shoot the bad guy for them, since they have more important things to worry about (donuts, anyone?)
7. For the worst case, have cameras & a time-lapse DVR running off a UPS in a secure fireproof vault. No matter what happens, you'll have lovely pictures for the cops & the news, so the bad guys will probably get caught.
Pay attention to the above and you should feel quite calm when away from home.