The PC industry is changing. Dell are very vulnerable right now and they are expecting bad quarters this year. As is Microsoft. The two are considering consolidating so they can focus on a single vision for the post-PC era.
This has everything to do with next-gen hardware and PC form factors (i.e. tablets) than anything else.
I'd not be surprised to hear soon that Intel are getting on this deal too. Since Intel has just posted major profit declines as a result of poor PC shipments worldwide.
I get the whole post-PC era thing, but why buy (admittedly they're not) something as big as Dell which has so much that MS wouldn't want.
What appears to be happening though is not an acquisition, as posted above.