I think, understandably, that there is a lot of confusion surrounding these laws (ie the European cookie law, as implemented variously by different European states).
From a UK perspective, I believe that we are actually already subject to the new law. The government has chosen, however, to allow a one year grace period (which we are currently part way through), during which time webmasters should be able to show (if challenged) that they are making demonstrable efforts towards implementing changes to their website(s) such that they will be fully compliant with the law by the time the grace period ends.
Also, I think that discussions about adding a permission statement within a forum's registration Ts and Cs are, with respect, missing the point. It's my belief that the law (as implemented in the UK) requires a user to be given the opportunity to opt out of cookie use at the very moment that they arrive on your site. This applies regardless of whether they are an existing forum member or just an unregistered visitor who is passing through (and wishes to read some of the forum posts before they leave, but has no intention of registering and no need to read your Ts and Cs).
As the enforcer of this law in the UK, the Information Commissioner's Office has implemented a pop-up permission box that appears as soon as you arrive on their site: www. ico. gov. uk (remove spaces).
I have various projects for which I want to use forum software. With the clock ticking down on the grace period, it's imperative for me that whatever forum I use includes an admin option that triggers an automatic permission gatherer like the one on the ICO's site. It also, obviously, needs to allow the user to either continue using the site without cookies (with restricted functionality, if necessary, like not being able to join) and that spells out what any "strictly necessary" cookies are used for and how.
As, presumably, a US-based project, I don't know whether Simple Machines is prepared to add this kind of functionality. If not, they will be severely restricting their market as far as law-abiding UK and European webmasters are concerned. In the long run, I could easily see how a lack of interest / willingness to address this issue could easily lead to SM getting a bad reputation and even to forum publishers suffering quality score penalties in natural search rankings.