In the words of Ken Mingis, Computerworld's managing news editor, Yahoo is becoming Moby Dick to Microsoft's Ahab. While Ballmer and team are obsessed by the fish that got away -- Internet search and ad sales -- Google is slowly plotting its way into Microsoft's enterprise business.
I would say that Microsoft's mistake is deeper than just an obsessive quest to own Yahoo. If and when Microsoft acquires Yahoo Microsoft will not make enough money from Yahoo or from the combined Yahoo-MSN to justify the cost of taking over Yahoo. Microsoft would be just as well off to invest their money in government bonds.
I don't see any reason for either Google or the open source community to expend any effort opposing Microsoft's attempt to take over Yahoo. Whether Microsoft takes over Yahoo or not will make no difference in Microsoft's financial strength or in their competitiveness in any market.
So if Steve Ballmer wants to chase Moby Dick I think that is harmless enough and no worry to anybody other than Yahoo.