Some new pacemakers [are MRI compatible][1]. Of those, some have an exclusion zone where the body may not be scanned. I wouldn't be surprised if, in a few years, all new pacemakers and ICDs will be MRI compatible. Older pacemakers still won't be, however. The MRI fields present at least two problems in pacemakers:

1. They can induce significant currents in the leads. This may cause tissue heating. The amount of induced current depends on the geometry of the lead, and likely other factors as well.
2. They can induce current in the pacemaker circuitry itself. This can cause the pacemaker to malfunction.

There may be other issues that I'm not aware of. But for at least these reasons, each pacemaker system (pacemaker and leads) must be tested for MRI compatibility and certified as MRI compatible (by the FDA, TÜV, etc.).


  [1]: http://www.medscape.com/viewarticle/780430_4