National Electrical Code 2014
Article 352 - Rigid Polyvinyl Chloride Conduit: Type PVC
352.10 Uses Permitted. The use of PVC conduit shall be permitted in accordance with 352.10(A) through (H).
(F) Exposed. PVC conduit shall be permitted for exposed work. PVC conduit used exposed in areas of physical damage shall be identified for the use.
(G) Underground Installations. For underground installations, homogenous and nonhomogenous PVC shall be permitted for direct burial and underground encased in concrete. See 300.5 and 300.50.
Article 300 - Wiring Methods
I. General Requirements
300.5 Underground Installations.
(D) Protection from Damage. Direct-buried conductors and cables shall be protected from damage in accordance with 300.5(D)(1) through (D)(4).
(4) Enclosure or Raceway Damage. Where the enclosure or raceway is subject to physical damage, the conductors shall be installed in rigid metal conduit, intermediate metal conduit, Schedule 80 PVC conduit, or equivalent.
352.10(F) says that the PVC conduit "used exposed in areas of physical damage shall be identified for the use". I know schedule 80 is suitable for protection from physical damage, however, the Authority having jurisdiction in your area may allow schedule 40 as well. You'll want to check with your local inspector to be sure, or use schedule 80 for the exposed section.
300.5(D)(4) says that even though the conduit is buried, if it's subject to physical damage (not deep enough, under a garden, etc.) it still has to be schedule 80.
Schedule 40, 80, Type EB and A Rigid PVC Conduit and Fittings
1.2 Schedule 40 and 80 conduit and fittings
1.2.1 Schedule 40 rigid PVC conduit and fittings are for aboveground use indoors or outdoors exposed to sunlight and weather, and for underground use by direct burial or encasement in concrete. Schedule 40 rigid PVC conduit, elbows, that are specifically marked for underground use are suitable for use underground only by direct burial or encasement in concrete.
The UL listing says both 80 and 40 can be used in both above and underground installations.
Guide Information for Electrical Equipment The White Book 2013
Rigid Nonmetallic PVC Conduit (DZYR)
Use and Installation
Schedule 40 conduit is suitable for underground use by direct burial or
encasement in concrete. Schedule 40 conduit marked "Directional Boring""(or "Dir. Boring") is suitable for underground directional boring applications.
Schedule 40 conduit is also suitable for aboveground use indoors or
outdoors exposed to sunlight and weather where not subject to physical
damage. Schedule 40 conduit marked "Underground Use Only" is only
suitable for underground applications.
Schedule 80 conduit has a reduced cross-sectional area available for wiring
space and is suitable for use wherever Schedule 40 conduit may be
used. The marking "Schedule 80 PVC" identifies conduit suitable for use
where exposed to physical damage and for installation on poles in accordance
with the NEC.
The White Book clearly states that only schedule 80 is suitable for use where exposed to physical damage, though again, says schedule 40 is suitable for aboveground use (unless labeled otherwise). So while you may be able to use schedule 40 for the entire run, you'll have to use schedule 80 through any areas where the AHJ deems the conduit is exposed to physical damage.