ThinkSecret publishes a rumor that Microsoft is working on MSN for Mac:
According to one tipster, the company is currently beta testing MSN Internet Access for Mac, in cooperation with Qwest Communications. The test is not being handled by Microsoft's Macintosh Business Unit (which developes Office and Internet Explorer) but rather by the company's MSN division.
This corresponds with a rumor submitted by coredump to Macrumors.com a number of days ago:
I work for Microsoft at the Silicon Valley Campus in Mtn View. We are currently testing MSN for the Macintosh with Qwest.com. This test is being conducted by the MSN division, not the Mac BU.
This is likely the same source who submitted information to both sites.