In the 2008 NEC, 352.10(G) (PVC Conduit section) refers you back to Table 300.5 (minimum cover requirements, 600V and less). The burial depth of the pipe will depend on what's above it. Table 300.5 Column 3 should give you the answer for PVC.
The area I will be burying the conduit is on a grassy area beside a parking lot. Because I do not see anything covering a "field" or "grassy area" I believe that I should use the first row in this table for "all locations no specified below" and therefore burry it at least 18" deep
...and therefore burry it at least 18" deep
A minor detail. It's 18" of cover, not 18" deep trench (I know you didn't say this).
Chris it's easy for you, you have Al.