I'm up for it - weekly or monthly.
I'm in a photographers group on LJ, and they do monthly 'challenges' (really, themes) such as movement, water, a specific colour, etc. So while the category could be very general, it help to really look at/for things to shoot.
As far as rules, I'd say allow the theme to help you find something, but if you take a photo of something that you love, then post that! Even if it's nothing to do with the theme.