I had heard from this initiative internally a while back, but it is now official: users can comment/annotate some MSDN reference pages. More info can be found on the MSDN Lab Annotations page.
The Longhorn SDK pages on MSDN already have the feature deployed.
It looks very similar to the annotation feature in the PHP documentation which is most useful. But there are some differences:
- the MSDN annotations appear to only be available in IE,
- they are exposed via RSS feeds, which I don't think is the case for PHP notes,
- there is no mention of any moderation, while the PHP notes are moderated.
Anyway, I couldn't find any annotations on the API pages yet, so I guess we'll have to wait to see if this feature really enriches MSDN, as it did PHP.
Update: O'Reilly has published annotations for the Longhorn SDK and describes the annotation system. Annotations can be provided by third-parties, through an specially-formatted RSS feed (fetched, cached and displayed by the SDK annotation component that you install on the client).
Update: Thinking more about it, RSS seems like a bad solution to the annotation problem. What you want is some kind of annotation database that can be queried for information related to a specific page or topic, not a stream of annotations. You still want to access old annotations, but they'll get pushed out of the RSS feed as new annotations are added...
On the other hand I agree RSS fits in the scenario where you want to monitor any new notes posted relative to a topic, but I would guess that's not the most common need.