I think the biggest challenge they will encounter is the need to re-jigger the notion of what Google does in the minds of their users. When somebody says "Google" a non-techie person will think "internet search engine".
I think it's going to require a bit of time/marketing/something else to reshape the notion of Google as broad internet search engine into "it also searches your personal stuff and also your friend's stuff"
Its hard for me to imagine they'd want to do that at all. The average non-techie only knows about Google because it means "internet search engine." Any complication of this will only bring confusion.
You have to wonder how Bing will respond. Not because they must, but because it's inevitable that they will replicate any major Google Search features...
I think it's going to require a bit of time/marketing/something else to reshape the notion of Google as broad internet search engine into "it also searches your personal stuff and also your friend's stuff"