I got the following from here:
http://www.irs.gov/publications/p378/ch01.html#d0e530
Nontaxable Uses
The following are the uses of undyed diesel fuel and undyed kerosene for which a credit or refund may be allowable to an ultimate purchaser.
<UL TYPE=SQUARE>Off-highway business use.
Export.
In certain intercity and local buses.
In a qualified local bus.
In a school bus.
Used other than as a fuel.
In a train (undyed diesel fuel only).
In a highway vehicle owned by the United States that is not used on a highway.
Exclusive use by a nonprofit educational organization.
In a vehicle owned by an aircraft museum.[/list]
No credit or refund is allowable for any use of dyed diesel fuel and dyed kerosene. You buy these fuels excise tax free. See Publication 510 for more information about dyed fuels.