This inter-board PM is a great idea!
OK, I've got some ideas of my own that I want to tell you guys and I hope they haven't been said before because I only read the first few posts....
Some people mentioned spam between boards and stuff and I think I have a solution.
Think of a 'key'
Scenario: Board A and Board B both have this mod and wish to exchange PMs with it.
First, Board A's admin and B's admin make a key, which could be any random string, such as 'qwerty'. The string is hashed via MD5 and stored in the database. Both A and B do this.
Next, A and B admins tell eachother what their secret key is, but not MD5, they have to tell that the key is 'qwerty' or whatever it is.
Now, if a board wishes to participate, it must know the key in order to receive messages. Each message sent by B to A will have the key in MD5 format. It is then compared to the master key in A's database, and if it matches, then the first check is done. The second check is that the key from the other board is checked. Essentially, you have the two boards verify eachother's secret key to make sure that the message is legit. A potential spammer would have to know both keys to make a form.
The two admins may change the keys if they wish, and that may deter spammers.
That's my idea, and if anyone has comments on it, that would be great, since I don't think it is 100% fool-proof.