When composing a message, you'll see the new DICE bbcode which cab be used to roll dices!
This Mod has all the basic math (add/subtract/multiply/divide) so you could have something like 10/(3d20+3)*-2.5) within the dice BBCode. One limitation of this module (and this is actually the ONLY module currently available to roll dice in PBPBB3) is that you can't have [at this time] multiple dice like 1d4+3+2d6, which would have been useful for a Rogue, for instance.
You can test the dice bbcode in the DICE BBCODE Testing Area! thread!!