There are many foundations offering awards for established entrepreneurs (read: Ernst and Young, Am Ex, Eileen Fisher, others). Below, however, is a nice monetary prize for a university level plan. (Also see Association for Corporate Growth hosting a student business plan competition for $10k in DC in 2008).
The Queen's Entrepreneurs' Competition The Queen's Entrepreneurs' Competition is an international business plan competition that allows university students from around the world to unite, present their innovative ideas and develop their entrepreneurial skills, with a chance to win over $25,000 in awards. Submission deadline is December 12, 2007. Click here for more info.
Also, here is the Rice Business Plan competition – one of the larger graduate-level business plan competitions. This past year they awarded almost $350K in cash and prizes. Part time grad students are eligible for you Hot Mommas who are really doing it all (school & work). Click here for more: Rice competition.