Children's "appetizers" are not the same as adult appetizers. Children must order off the child's menu and pick the courses specifically from that menu. They can't order appetizers from the "adult" menu.
The only ways for adults to get appetizers on the basic plan are:
Buffets, or all you can eat family style restaurants like Ohana or CRT, or dinner shows (the dinner shows also include alcoholic beverages -that is the only way to get the dining plan to cover an alcoholic beverage)
The Fantasmic Dining Package at Mama Melrose or Brown Derby
The "pilot menu" at Le Cellier
Marrakesh allows basic dining users to order multi-course meals with an upcharge.
That's it. Substitution of appetizers for desserts on the basic dining plan is strictly prohibited by WDW.
for the OP - If you haven't seen it already, take note that the 2008 dining plans also do not include gratuities at table service restaurants, so it's necessary to budget to pay your own tips.