when you look at http://www.microsoft.com/exchange/2010/en/us/Licensing.aspx
you will notice that standard edition can have 5 databases. this limit should inclued Public folder database and i think everyone will understand that. But how about DAG’s passive and active databases? Do not misunderstand! The 5 databases limit is a sum of [passive mailbox databases + active mailbox databases + public database]. For example, you can have only 2 active mailbox databases and 2 passive mailbox databases and 1 public databse on one Exchange 2010 standard edition server.