I realise that it's not the most reliable source, but I read a rumour [1] that Apple offered $800m to acquire Dropbox and they declined. It's possible it simply wasn't enough, but perhaps they're not interested in selling.
Do you see MS putting Amazon in their critical path? I easily picture Ballmer saying NFW, but would love to hear why MS would hand such power over to Amazon.
They're not exactly mortal enemies, other than when it comes to recruiting Seattle talent. Amazon's pricing is transparent and MS would probably plan to migrate it to their own data centers like they migrated Hotmail from Unix infrastructure, and probably lots of other products.
...which tells you the only logical acquirer. Any other will have a middleman of S3 to pay, which will make the financials not very sensible.
If GDrive releases and is viable, Dropbox should put on their best push-up bra and fishnets, and head over to seattle as soon as the mascara is dry.