Here is one idea which i think is VERY good, and if you do decide to implement it, i would [red]very[/red] much like to help you with it.
Implement a yahoo-messenger-like PH messenger to be used for messages!
if the user is online, he can get alerts like [blue]'3 New replies to your post[/blue] "[red]How do i use interrupts[/red]" at [blue]'C/C++ Message Board'[/blue] or a better way, columnized according to the boards and messages !!
If you ever decide to host a e-mail provider, the above idea can easily be extended !!
I'm already in the process of defining the blueprint for just such a software, and if you're interested, let me know at email@example.com