I have 2 exchange 2010 mailbox servers. I use DAG to replicate the mailbox-content. so 3 mailbox databases are replicated to the other exchange mailbox server. question? is it allowed to create a public folder server on 1 or 2 of these mailbox servers, or do I need a third exchange server ? (I'm aware that PF uses its own replication mechanism. I'm interested in the question if it is allow/supporte to have PF database on those mailbox servers ?

Yes it is supported, you don't need a 3rd box. You can host public folder databases on both DAG members and use public folder replication.

“A public folder database can be hosted on a Mailbox server in a database availability group (DAG), but you must use multiple public folder databases and public folder replication for data redundancy” -----------Refer to <Understanding Public Folder Replication>James Luo TechNet Subscriber Support (http://technet.microsoft.com/en-us/subscriptions/ms788697.aspx) If you have any feedback on our support, please contact tngfb@microsoft.com