The Foundation thought about this, and even did it during the fundraising drives (see http://wikimediafoundation.org/w/index.php?title=Fundraising_FAQ_2007-2008

The Foundation thought about this, and even did it during the fundraising drives (see http://wikimediafoundation.org/w/index.php?title=Fundraising_FAQ_2007-2008&oldid=18436#How_is_this_different_from_a_company_receiving_ads_.28i.e._logos.29_if_they_pledge_to_donate_money.3F http://wikimediafoundation.org/w/index.php?title=Fundraising...). The most (in)famous example was Virgin Unite.