In regards to articles I believe SitePoint is going for quality over quantity. So, ideally, each article should be quite in-depth even though a new article isn't published every day. The blogs also provide daily updates.
If you're interested in writing an article for SitePoint then, as iFroggy said, you should contact them first to check which article topics they are interested in. email@example.com.
That is one reason why I prefer SitePoint to DevShed. DevShed articles are crap, short, and there seems to be many duplicate topics - even if there are new article daily. I'd like to see at least three articles a month from SitePoint, but there hasn't been an article from SitePoint in quite some time.
I submitted three URL's of articles. I don't think that they're SitePoint quality, but if they're interested in the topics I'm willing to expand and go more in depth.