Will the “Petition Grant” letter be available in PAIR for the auto-granted petitions?

Yes, the "Petition Grant" letter will be available in PAIR on the second page of the acknowledgement receipt. Current auto-grantable petitions include the Petition to Accept Unintentionally Delayed Payment of Maintenance Fee in an Expired Patent and the Petition to Make Special Based on Age.

USPTO FAQ - Questions and Answers (2008MAR10)