This work is supported in part by NSF grants EIA-0101247, CCR-0205594, CCR-0237113, CNS-0509447, DMS-0528414, NSF-0101247 and by research grants from Google, Intel, Microsoft, and Yahoo!.

*Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.