I will be developing it in PHP, and once I get a framework set up I'd be glad to let you help out. Especially the "posts since you were last here" function.
Sure, though I suspect I could contribute a bit more than just that. Thats a pretty simple thing to code.
I'll briefly throw out one of my own ideas for a PbPRPG forum:
One thing that I think would be important for a forum like that is Character Profiles. In other words, members sign up with ONE member ID, and then can have up to X character profiles, with independent stats, bios, pictures, signatures, inventories, and so forth. Also, it'd be cool if Character Profiles could have Alter Egos, for the purposes of Disguises, Secret Identities, etc. Alter Egos have different pictures, names, sigs, etc, but share the same stats, inventories, and backgroud info since they are technically the same person.
You could do this using an SQL table for Members, one for profiles (with an ParentMember ID column), and AlterEgos (with a ParentProfile ID column).
Member 1
|-- Character Profile 1
| |-- Alter Ego 1
| |-- Alter Ego 2
|-- Character Profile 2