I see no reason for the government to stop this.
Google is still a very strong rival to Yahoo, and Apple has consecutively posted great numbers in the last 6 quarters. They are not coming close to any monopoly in any industry.
I think this is a great thing for Microsoft, and I'd really like to see some cross-coordination in creating some new programs. Perhaps Yahoo mail will start supporting POP3 so I can make use of Outlook :-D