As we reported yesterday, Microsoft is continuing to grab market share in Internet search. It has done that through some crafty marketing efforts, the latest of which is a partnership with DonorsChoose.org to help support school programs. Microsoft will offer a $3 donation every time someone pledges to a school project like the Teacher Appreciation program or the Earth Day Photo Contest. Sounds great, huh?

Well, there's a catch. In order to enable the $3 donation, users must set Bing as their default search engine.

Here's how it works. Potential donors go to the DonorsChoose.org school project page where they are prompted to "Choose bing and donate." After providing an email address, donors are given a promotional code for the $3 Microsoft donation to assign to the project of their choice. The money goes to literacy, music, math and other school programs.

Microsoft said it will donate up to $100,000 through the promotion which is set to expire in late August, so not sure this will drive huge market share gains for Bing this summer. But it does speak to the creative ways that Microsoft is looking to drive new usage of Bing as it battles Google.