It would seem people are bored on a Friday, so it's a good day for rumors of companies buying other companies. First it was rumors of Google buying Yell, now it's rumors of Microsoft buying Limelight Networks.
Apparently Microsoft is interested in the content-delivery network, which is in second place in the field, more for it's infrastructure and experience than their existing customer base. This does fall well within the announcement last year that Microsoft wants to purchase 20 companies a year. Seeing as the Redmond, WA-based company wants to spend between $50 million to $1 billion per company, and Limelight has a current valuation of $550 million, this falls well within their parameters.
Course, since this is a rumor, who knows if Microsoft has ever even heard of Limelight, but that's the way these things go.