The optional speed bump now will allow you to get the fastest ego stroking machine now and for Apple to make use of a more limited supply of chips.
If you want a 2.6GHz machine you can pay the extra money now and get your grubby hands on it several months before those waiting for the next generation of CPUs will.
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Of course if you wait, the cache will be bumped in the next generation CPU -- which will likely help out more than a simple GHz bump.