Not sure how new this is because I don't spend a lot of time on LinkedIn - I am generally equally happy to introduce myself to someone cold as I am to contact a '3rd degree' person.
Spidering Outlook (or webmail) is starting to look like table stakes for any social network save the top tier winners like Facebook. See Techcrunch coverage of Hi5 as an example. ***Update*** Facebook also has a contact spider feature.
I don't have much to say beyond that this feature will rapidly expand LinkedIn signups.