Kari,
As long as the table service meal did not require a credit card deposit, you will be fine if you do not show up for your meal reservation. As a courtesy to all other guests, you should call Disney Dining (407-WDW-DINE) to let them know that you will not be able to make your reservation. That way they will be able to accommodate other guests at the restaurant more easily.
Keep in mind, though, that if you don't find an acceptable substitute table service reservation, you might end up wasting that extra dining credit. This is one issue to be aware of with the Dining Plans -- they work great and usually save you money if you can use up all of your credits. If, however, you end up skipping table service meals or not being able to get reservations for enough table service meals, the dining plans don't end up being such a bargain. It is not uncommon to end your vacation with leftover credits if you don't plan carefully. Fortunately if you have snack credits that are leftover, you
can use them on your last day to get souvenir candies at many of the stores. We bought tons of Mickey Mouse lollipops on our final day the last time we used the dining plan. ;-)
Just in case you haven't seen this, I'll also mention that you can click
here for a printable copy of the 2008 Disney Dining Plan Guide. It has lots of very handy info!