I think I might have found the answer. It appears that a seperate ground rod is required at the other building, however, a 3 conductor + ground service feed must be used.
Here is a diagram of the particular section of the NEC.
The following text is quoted from:
http://www.iaei.org/subscriber/magazine/03_f/03_f_threechiefs.htm
Subpanel located in or on a building or structure separate
from the service equipment
Section 250.32(B), Grounded Systems, contains provisions which result in two different ways to connect grounding and bonding conductors at a subpanel located in or on a building or structure supplied from a
separate building or structure.
Figure 2 illustrates 250.32(B)(1), the first way to wire a subpanel. The following are key points for this installation:
1. The subpanel is located in or on a building or structure supplied from a separate building or structure.
2. The subpanel is supplied by a three-wire plus ground feeder (two ungrounded (hot) conductors, one grounded [neutral] conductor and one equipment grounding conductor).
3. The neutral bus is isolated from the subpanel by insulating material.
4. A grounding electrode is required. Existing electrodes must be bonded to the equipment grounding terminal bar by a grounding electrode conductor sized according to Table 250.66 and based on the largest ungrounded (hot) conductor of the feeder (note the change from the 1999 NEC, where Table 250-122 was referenced).
5. Water and gas pipes are also required to be bonded to the equipment grounding terminal bar. See Sections 250-104(A)(3) and (B) for the bonding conductor size.
