A self-employed individual had a SEP in 2005 and will be incorporating as of May 1, 2006. Can he contribute to his own SEP for the first four months of '06? Can the corporation then start its own SEP as of 5/1/06 and make a full contribution for him for the remainder of the year?
Or is one SEP all that is allowed per year?

The EMPLOYER is the same. Merely changing the entity type does not create a new employer. You're overthinking it.