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Which is apropriate, with the spread of online apps and decent cross-platform desktop apps, the OS you choose will become irrelevant (eventually, might take a decade or so), and with Microsoft's and Yahoo's recenet track record of success *ahem* online, in ten years or so, the younguns' will be saying:
Think: MSN Yahoo!, Yahoo Live, etc.
Domains will be unchanged; expect the biggest winner here to be Yahoo.
Either way, this is not about merging and consolidation, it is a crude game of retaining traffic. Microsoft has constant streams of new user traffic but can't keep it, Yahoo! can keep new user traffic but they're not able to attract it.
Every new PC, every new download of MSIE, every default installation of MSN Messenger, etc. results in a browser homepage set to Microsoft's MSN service. That's massive and is where they get almost all their traffic.
Users get quickly frustrated and change their homepage to Yahoo or Google, depending on which they think of first.
Now imagine if MSN was a half-reasonable service with a reputable brand name, such as Yahoo. They could keep many of those default homepages. All the practically-stolen traffic might actually pay off in terms of producing serious userbase growth on the Yahoo! side.