I nominate Cathy Marshall, Senior Researcher at Microsoft Research. She's
working in personal digital archiving and personal information management.
I've been really impressed with two of her recent projects: a talk at ASIST
'08 about myths of digital archiving  and her JCDL 2009 paper "No bull,
No spin"--which examined zillions of flickr photos of a particular
tourist attraction to understand how they used tags.
Despite being a serious researcher, Cathy is very down-to-earth (check her
twitter feed http://twitter.com/ccmarshall or her older and sometimes
bizarre hypertext art). I think she'd be a great keynoter for Code4Lib.