Actually there is a logical reason why the Obama campaign would want to black out the number. By not releasing the number they make it impossible for a Photoshop artist to produce an altered Obama birth certificate and claim it is the real one.
If the number were not blacked out, all Obama would have to do is instruct anyone who was in doubt as to which one was real to send a copy of his (including the certificate number) to the state of Hawaii along with $5, and Hawaii would send back confirmation staying that it was real (and thus any other certificate that disagreed with it was fake).
In short, blacking out the number has exactly the opposite effect from what you claim.
No that is lame, because if there was a number, ANYONE could request a Letter Of Verification for all the information on the certificate. The numbers are not "secret". They start out 151 YYYY NNNNNN, with NNNNNN being the number of births up until that point in the year.