I'd say imposing a limit would be pretty much worthless. If someone can come up with more than 25 useful posts a day, they deserve the points they get. People who spam can be banned or deleted, which would be a good way of weeding out the troll types anyway.
Cecil's idea of limiting points recievable per whatever for each thingy [concise and meaningful ] really appeals to me. Although for articles rather than saying one per day, I'd probably say seven per week (or a similarly equivalent distribution of whatever is considered fair) as I can imagine having two ideas for articles together, or as many people do, a small series of articles, and it would be a pain in the ass to be semi-forced not to release them on the same day, when you probably aren't going to be writing any more for a small while (if you see what I mean).
Also, I personally wouldn't have given full credit for backdated points, but that is just me =]