I doubt the marriage will be a big threat to google. They have to come out with better services and products to combat google.
If it isn't Google won't be doing so much over this merger, it will have a fundamental effect on the Internet if this merger goes through, Microsoft and Yahoo are giants, though Yahoo isn't performing that well recently, a lot of their under performing assets can be improved upon and this will give Google a run for their money.
I hope that Microsoft's buyout of Yahoo does not occur. Yahoo has a lot of quality resources on the web, and I would hate for Microsoft to Microsoftize any of those sites. Microsoft often makes a mess websites.
compertition is always good, and I think with yahoo.. microsoft can compete well with google but.......... yahoo will all be recoded to ASP.net and PHP ditched oh, wait, no... php.net (lol) or whatever its called will be on the next ISS (or is it alreasy? do theyll be fine)