I am wondering (hoping) someone else has found a way around this same problem and can help. I believe Symantec would say to scan each PST first for errors as best practice but obviously this is not scaleable when you are talking about thousands of PST etc.
An example of the mig report is as follows;
Number of folders processed: 15
Number of items archived to vault: 19540 of 19620
- Number of items not eligible for archiving: 80
Number of items moved to mailbox: 0 of 0
PST Migration completed successfully
11/02/2009 20:47:26 Migration of \\servername\g$\Users\Archive.pst has been successful, but the PST file contains items that could not be migrated. The PST file has not been set read-only. The PST file has not been set hidden.
11/02/2009 21:07:07 Completed migration of PST: \\servername\g$\Users\Archive.pst