First of all, I would say, it's really too early to think about HTML5, while it's specification is not ready yet.
Second, I don't see any major benefits of switching to HTML5 even if it may be just about changing one tag. The most important with the current version of miniBB, is that it surprisingly quickly works on any browser. The major goal is achieved. Then why would we change it again and again and again? I don't think it matters a lot to the end user what is inside of the source code. May be only 0,05% of all miniBB end users know its architecture and code. For all other users, it's just a tiny and fast forums interface on the screen.
For me as a developer, it doesn't matter which technology is used to create a simple and stable program. I am working not for W3C and not for coders, I am working for the end users.
As about HTML5 benefits, in general. I know it brings some extras regarding multimedia content embedding, f.e. on YouTube they don't use Flash anymore, it's completely HTML5 which allows to display video streaming content. Also one thing appears on file upload forms - the "browse" field allows to select multiple files - but that's only half of the story, 'cause handling multiple file uploads requires uploading indicators, and that thing is not embedded in HTML5 and still brings lots of pain in the ass while figuring out how to do that.
None of the features above is related to miniBB. So I am not going to switch to to HTML5 right now.