After putting on both breakfast and lunch fundraising events, it really depends on your target market. For example, for one of my campaigns we had a lunch kick off open house (most were govt. workers and had the open house in a downtown location where the state buildings were located). It worked well because they could stop by, pick up their packets, eat lunch and be on their way.
In my former job, I also worked with a large-scale walk event and we had a Corporate Kick-off Breakfast early in the morning (very brief program--7:30am-8:30am) and we could always get a large crowd because the program ended in a timely manner (before their work day started).
In my experience with fundraising campaigns, it depends on the type of event (walk, gala, golf tourney etc), your audience, and venue. I'd suggest researching your donors' demographics and one hint--research other "competitive" NPO events in your area and make sure you don't have your event on the same day etc. That will make or break your event--no doubt!