Since Google walked away from its partnership with Yahoo! earlier this week over concerns about anti-trust hassles, Yahoo has been without a white knight investor to haul it out of its current financial hole.

Yesterday, Yahoo! CEO Jerry Yang told anyone who would listen, at the Web 2.0 Summit in San Francisco, CA, that Yahoo! is now for sale to the highest bidder. And Yang says Microsoft (MS) should buy it — but the price has to be right.

“”I don’t think that is a bad idea at all, at the right price whatever that price is. We’re willing to sell the company,” Yang told reporters.

Almost six months ago, Yahoo spurned an unsolicited offer from Microsoft to purchase the company for (US)$33 per share. At the time, Yang and the Google board thought they could do better. MS backed out of talks saying it wasn’t prepared to pay more for Yahoo!, which was already in financial difficulties. That was when Google stepped in — some said, to tweak MS’s corporate nose — and opened talks with Yahoo! on a partnership involving search and ads. That deal remained in limbo, while Google accountants and lawyers mulled it over, until this week, when Google backed out.

Industry observers say MS probably still wants Yahoo!, but at it’s price. And they suspect MS CEO Steve Ballmer is taking some pleasure in making Yang and his board squirm a bit before even sniffing at their bait.