iMessages are linked ti the ID that was used.
On the iPad they are Gone.
If you have a Mac then in Mountain Lion OS X 10.8.2 you can use the same Apple ID
Then in the Preferences > Messages you can choose to Save History on closing the conversation.
The Chats/Conversations are then Saved on the Mac (in ~/Library/Messages/Archive)
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously