What this stock price says to me is that investors are betting that this story, even if it's correct, is not the last word, and that Microsoft (or someone) will eventually get them.
If Yahoo! does reject the offer, their stock price should, were the market completely rational, fall a lot. Back to what it was before Microsoft made the offer in the first place.
Yahoo's the same company it was two weeks ago; the stock was just worth more since MS was offering a premium.