I don't think sending out a primary with a score under 20 is a good idea at all, even if you do indicate on there that you're rewriting. 27 or smth is a different situation, but 20 is a pretty dismal score (sorry, being straightforward, and I'm sure you know this yourself already).
DON'T rush into taking the MCAT as early as possible to apply this year....better wait another year, prepare your butt off, get a 30+, and get in for 2009 than to take it this July, get a 24-26 or something else unacceptable along those lines (unacceptable for a rewrite), get rejected everywhere and have to go through it AGAIN. And next time it will be even harder - the more you rewrite, the more suspicious you look. E.g., they might start thinking "how come this kid is unable to apply him/herself completely on his/her first try?" Nobody wants to accept someone who can be reasonably expected to bomb the USMLEs over and over 2 years later.
Take the time to prepare to give your best. The world is not going to come crumbling down if you don't enter in 2008. I'm sure you could come up with SOMETHING to keep yourself busy for an extra year.