From ehow.com:

Add fruit to a cake mix for a new, exciting flavor instead of plain old cake. Be forewarned, however: there is a trick to adding fresh fruit to any cake. It can't be done at any old time. Otherwise you'll end up with a mushy, soggy cake. The fruit muse be carefully added at the last minute.

Instructions:

Wash the fruit and place on paper towels to dry.

While the fruit is drying, mix the cake as directed on the packaging.

Allow the cake mix to sit a few minutes to thicken.

Pour the cake mix into a prepared cake pan.

Place the fruit in sporadic spots in the cake mix.

Gently press down on each piece of fruit so it is submerged in cake mix, but don't squish it.

Bake the cake as directed on the package.

Keep an eye on the cake as it bakes - you may need to adjust the cooking time.