Toys for the 2004 storyline "Metru Nui" are showing up at various retailers. More information on Metru Nui should debut on the official Bionicle website in February!
Apparently, Burger King in the UK has had a Bionicle: Mask of Light promotion since November 24th... no idea whether it's still going, as I'm in the wrong geographical location to check. :(
And finally, for anybody with younger siblings (or a taste for the medieval):
Bionicle now has a companion line called "Knights' Kingdom". Like Bionicle, "Knights" features dynamic action figures that you build yourself. On the website, you can see Episode 1 of the Knights' Kingdom comic, as well as all kinds of background information on the series, the history of knights, and of course the action figures themselves.
I'm kind of hoping that the Knights' Kingdom figures will be compatible with Bionicle, so that we have even more parts to work with, but it looks like they might have totally new parts and a new system of construction.