OEMs like Dell, HP, and Lenovo will sell the majority of Windows 7 licenses along with the computers they sell. That puts these companies in the driver's seat, because they can configure the systems to use whichever browsers, search engines, and other Internet services they want. Normally they want whatever makes them the most money. Make no mistake about it, most OEMs will be happy to reconfigure the browser search engines if they're paid to do it and Microsoft is hungry enough to buy Bing search share. OEMs would be overjoyed to have Microsoft and Google start a bidding war over default browser settings.
You may be thinking, "What's the big deal, since users can easily change their search engine?" True, it's easy for technically astute PC users to select a different search engine. However, there are a lot of non-technical users out there. If they change PCs, they often use whatever happens to be set up on the computer by the person or company that set it up. Others use the settings from a toolbar they downloaded that changed their default search engine, or make do with whatever their ISP's "support software" installed. Those are generally "pay for play" deals as well.
If Microsoft does decide to buy themselves a bunch of market share by paying for OEM default settings -- and I think they will -- expect to see lots of news stories about Bing's rising market share. However, don't mistake that as an enthusiastic embrace by PC users. That kind of market share needs to be earned and not bought.