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 [1] and her JCDL 2009 paper "No bull,
No spin"[2]--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.

Her homepages:
http://research.microsoft.com/en-us/people/cathymar/
http://www.csdl.tamu.edu/~marshall/

[1] http://research.microsoft.com/en-us/people/cathymar/asist-marshall.pdf
[2] http://research.microsoft.com/apps/pubs/default.aspx?id=80591