Both companies are in trouble now, and they will stay to be once purchased (if it goes ahead).
It's great for Google because Microsoft will likely throw a load of cash at it, and get nowhere. Also, there are some great employees who work for Yahoo that may not want to work for Microsoft. Some of these may go to Google.
Yahoo have long been a supporter of free software projects and use software like freeBSD on their servers. If Microsoft gets its grubby hands on Yahoo, they may well start converting everything to .NET technologies and their own server operating systems.
I hope they don't do anything to del.icio.us or Flickr (both owned by Yahoo).