I spoke to an employee of the local Apple Store about this.
She said that if you purchase your MacBook within the two weeks prior to the rebate program's going into effect, you can bring the computer back in to the store, essentially trade it in for itself, and receive the iPod rebate along with the "new" purchase--with, naturally, no extra expense. I have actually pulled this off at another Apple Store, about five years ago.
The employee also indicated that the rebate program is likely to start at the end of May. So, if what she says applies to other Apple Stores, you should be able to purchase the MacBook by about May 16 or so, then return in a few weeks to claim the rebate. It's more of a hassle than simply waiting until June to buy the laptop, but if you need to own the computer a bit sooner than that, this policy might come in handy.