This article is from the Static Electromagnetic Fields and Cancer FAQ, by John Moulder email@example.com and the Medical College of Wisconsin with numerous contributions by others.
A broad range of whole organism and cellular genotoxicity studies of
static fields have been carried out. Together these studies offer
convincing evidence that static magnetic fields are not genotoxic.
Whole organism genotoxicity studies with static magnetic fields have
been somewhat limited. Beniashvili et al  found no increase in
mammary cancer in mice exposed to a 0.02 mT field. Mahlum et al 
found that exposure of mice to a 1000 mT field did not cause mutations,
and other investigators found a similar lack of mutagenesis in fruit
flies exposed to 1000-3700 mT [14,15,16] fields.
There has been one report of possible genotoxicity. In that study Koana
et al  found evidence for increased mutations in repair deficient
fruit flies exposed to a 600 mT field for 24 hours. No effects was seen
in fruit flies that had normal DNA repair capacity.
Cellular genotoxicity studies have been more extensive. Published
laboratory studies have reported that static magnetic fields do not
cause any of the effects that indicate genotoxicity. Static magnetic
fields do not cause chromosome aberrations [18,19,20,21,22,23], sister
chromatid exchanges [18,20,22,24], cell transformation [19,25] or
Some studies of static electrical fields have also been conducted.
These have been reviewed by McCann et al , who concluded that while
there were some reports of genotoxicity for static electrical fields,
"all reports of positive results have utilized exposure conditions
likely to have been accompanied by auxiliary phenomena such as corona,
spark discharge, and transient electrical shocks, whereas negative
reports have not."