I've given this subject some thought. I do know some magazines are loathe to accept materials which have already gotten significant play on the web since it reduces the selling point of that content. The flip side of that is that some layouts or subjects are better known because of their web presence, thereby potentially increasing the selling point.
All of that said, though, one really has to have an article submission in mind before one even starts. I'm happy to post iPhone grabs here when I think I've dun good, but that's very different than taking print-quality photos at every step in a project. The photo standards very by magazine, and when I did my N Scale Magazine articles I was using a crappy point-and-click that nevertheless didn't prevent me from submitting. Yet my current DSLR photography is still not up to MR's snuff.
Long story short, most forum projects don't readily translate to articles (although some do). Furthermore, if the entire article ends up free here for all to see, a magazine is less apt to want to pay you for it.
Lastly, <mod_hat> we've tried to make this forum more functional and more intuitively organized, and each time we've made a change, such a chorus of whiny bellyaching completely unbecoming of grown-ups rises up as to make one want to swing a shovel indiscriminately, so that's reason enough--independent of the aforementioned--to not do it </mod_hat>.