[vellum]

I don't think you need a unifying mission. Take Apple, for example. They just make great product lines.

Their mission statement [1]:

What is Apple's mission statement?

Apple designs Macs, the best personal computers in the world, along with OS X, iLife, iWork and professional software. Apple leads the digital music revolution with its iPods and iTunes online store. Apple has reinvented the mobile phone with its revolutionary iPhone and App Store, and is defining the future of mobile media and computing devices with iPad.

Yahoo could do the same by paring their offerings into a set of great products.

[1] - http://investor.apple.com/faq.cfm?FaqSetID=6

[Julianhearn]

Having a mission like "the best personal computers in the world" sounds like a pretty hairy mission to me.