If we get those 3 spots because both Rachael and Alissa go out there and completely nail it, then all they are doing is strengthening their own case for Vancouver. In addition to 2010 Nationals, performances at 2009 Worlds do weigh very heavily on Olympic team selections. So if they both nail it here, then there goes two out of three spots for Vancouver.
But if say Rachael does fine and Alissa melts down completely tonight (not unlikely), then that's not a very good statement for Alissa to be sending out. USFSA will think "look what happened the last time we trusted her..." and there would still be 1 spot open for Caroline, assuming Rachael solidifies her chances for spot #1.
And of course we don't know what the deal with Ashley and Mirai will be, but you get the picture.