Here's the EPAs writeup.

http://www.epa.gov/mtbe/water.htm#concerns

As in the famous case of Erin Brockovich, there is no evidence that MTBE or Chromium-6 is harmful in water. Both substanced have been found to be carcinogenic if INHALED directly in huge doses as opposed to the parts per billion levels we might see. Soon enough some lawyers will find a way of getting rich off us taxpayers because of MTBE, like in the Brockovich case. In her case it was fortunately only California residence, PG&E simply and legally increased utility rates to pay for the settlement.

In MTBEs case, in drinking water, at levels of 20-40ppb it becomes detectible by smell, kinda ruining it's use as drinking water for most of us, but there doesn't appear to be any health problems.