In a recent discussion amongst those on the WordPress Documentation mailing list, Scribu brought up the question on whether or not links to commercial solutions were allowed within the Codex. Lorelle VanFossen summed things up pretty nicely in the following quote:
A fantastic article that helps spell out the details of the Codex article on a site that isn’t selling “commercial content” other than hosting advertising and such (normal stuff) more than producing free, “open source,” shared content is allowed as it is considered external reference material. These are set usually in the “More Resources” section not within the Codex article.
Seems like a good compromise to me as it should prevent commercial entities from being able to use the Codex for link juice. Although that should be a lot more difficult considering new user registration for the Codex was disabled a few months ago to try and thwart spam attacks.