Best summary is on Read/WriteWeb:
To summarize: today LinkedIn announced some cool new features and made more promises about what will be a closed platform for select 3rd party partners, built on top of an API of questionable worth and targeted at business users with a history of disinterest in social applications. It’s not ready yet. Did we mention that the users will be people with money, though? I imagine that for that reason if nothing else, widget developers will remain very interested. I’d like to see LinkedIn do something better than this, though.
What it means: although the announcement is still a bit “vaporware-ish”, I think there is tremendous potential in tapping into the LinkedIn user crowd. If developers can build useful B2B-oriented apps, strong adoption could follow. But LinkedIn needs to open up the API to a larger developer base before it happens. Another developing story to keep on your radar screen but no short-term action.