There isn't really any need for it to be a plugin. It's fairly straightforward to deep integrate the two already. It's not particularly hard to create a bbPress theme grabs your WordPress theme header, footer and overall design either. My bbPress template generator is already able to do that with themes form my WordPress theme generator and there is no reason why WordPress theme developers couldn't add support for it within their own themes. It does require both the WordPress theme and bbPress theme to be coded to match up correctly, but if a few different bbPress core themes were developed, then WordPress themers could pick the bbPress theme which matched up with their own theme their own template files the best and recommend their users use that.

Adding bbPress support as a plugin would still be handy for some people though and I'd be surprised if it doesn't happen eventually.