The old formula was subtract your age from 220. This means as a 30 yr old my target heart rate based on this formula is 190. It's the target I aimed for when training for the half marathon, and it admittedly was pushing me a little too hard sometimes. I had a hard time maintaining that for an extended period of time.
The new formula is take 88% of your age and subtract it from 206. This makes my new target heart rate 180. 10 beats per minute isn't a huge difference, but it does add up. I can deal with 180 and still feel like I'm getting a good work out.
Will this change the way you train? Of course I assume you're a woman when I ask that. Men, keep using the old formula. I think this will be in more use once heart rate monitors begin using the new standard, but you can program it to watch out for a user-determined range.