Don't be intimidated by the "business casual" dress code - it doesn't mean the same at Disney as it does in corporate America. Pretty much anything goes except swimwear/cover-ups, flip-flops, tank-tops (on men, mainly) and clothes with holes. Decent parkwear is fine.
From Allearsnet's dining news page: Effective July 1, 2007, Disney modified its Business Casual Dress Code at its premier restaurants to include t-shirts. The guidelines, which originally went into effect July 2005, still prohibit hats, swim suits, swim suit coveralls, tank tops, torn clothing or flip-flops. Suggested attire includes dress shorts, pants, jean pants, collared shirts, blouses, sweaters. Restaurants now requiring Business Casual dress include: Artist Point (Wilderness Lodge), California Grill (Contemporary), Citricos and Narcoossee's (Grand Floridian), Jiko (Animal Kingdom Lodge), Flying Fish (Boardwalk), and Yachtsman Steakhouse (Yacht Club).