personally i still think weekly or fort...er two-weekly. especially if you get people post no more than 2 to 4 pictures. photoshop ok, interpretation ok. no contest.
as pointed out, there are people who are intimidated - especially by a contest/competition/being judged. but this should be something that everyone can participate in. it's all about learning - both more from a technical perspective, but also learning more about yourself. it's also about fun and challenging yourself not to be lazy. the thought of having 24-52 thought provoking opportunities per year to shoot something vs. 12 sounds much better. it's an opportunity for many people to come up with ideas as well.
there are many people who challenge themselves to do a photo-a-day gallery on pbase, flickr, etc. and the quality of their shots has gone way up. at this point in time i can't quite do a PAD, i can't but i've managed to do a photo-a-week gallery for over a year now. it's definitely a satisfying venture.
i do want to say i appreciate what handicap18 has done and i'm happy to participate in whatever comes about. if we want to continue the monthly assignment (which is already working), then maybe we might consider a separate thread that is themeless. a photo-a-week thread where each person can post no more that one photo taken that same week.