I believe I've seen the "D" for email that I've sent to another Blackberry user, which led me to believe that mail sent from a BB and then received on a BB are subject to delivery notification (like same-carrier SMS)...
The "D" means "Delivered," meaning the message was delivered to the mailbox, and you will get that for all PIN messages notifying you that the pin message was delivered to the BB. You can also get it with email too if you have delivery reports enabled in your Messages Options.
The "R" means "Read," meaning the person on the other end opened the mail. You will get this on emails if you have "read receipts" enabled. You will not get this when sending a standard pin message (wish they could enable this.)
Both of these notifications will automatically be received when using BBMessenger (uses pin messaging.)