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I looked through ebs.F to determine how vasp calculates the ewald energy. The expression appears to be E(direct) + E(reciprocal) - E(self interaction) - Zsum^2/(2*omega^1/3). This last term appears to be some sort of interaction with a uniform background charge, but it doesn't make sense to me. Doesn't the interaction of the ions and the neutralizing charge exactly cancel the G=0 term of the reciprocal sum?

I'll answer my own question. I found a good paper that derived the Ewald energy for ions in a compensating uniform background charge. I am not sure of the ref, but the title is "Calculating ... Interactions ... in a Periodic System with a Neutralizing Background Charge Density" and the author is Minghong G. Wu. You can get it from the following website: http://arxiv.org/pdf/cond-mat/0112138
Basically, this term accounts for the short range interaction of the ions with the uniform charge background charge. Although the long range (reciprocal space) term cancels, the real space term doesn't.