Katie,
In case you haven't seen it yet, the Minimum Program Requirements (MPR) Supplemental Guidance document provides some help here:

1. The LEED project boundary must include all contiguous land that is associated with and supports normal building operations for the LEED project building, including all land that was or will be disturbed for the purpose of undertaking the LEED project.
2. The LEED project boundary may not include land that is owned by a party other than that which owns the LEED project unless that land is associated with and supports normal building operations for the LEED project building.
See
http://www.usgbc.org/ShowFile.aspx?DocumentID=6473
for this and the exceptions for non-contiguous parcels on page 13.

Project teams have a certain amount of discretion is determining what's included in the LEED site boundary, as long as it's reasonable and meets the MPR guidelines. One key question is what areas are included in the project scope of work and being improved. If the driveway is existing and not being improved, while the parking area is being improved and supports the building operations, the drive could probably be excluded and the parking lot be included. Sometimes a property line is the most logical site boundary, but often it's the project scope of work or area being impacted.
Just be sure to address all credits for all the areas included in the LEED site boundary, such as stormwater, light pollution, heat island effect, construction waste, material costs, recycled content, etc.