We were in Aruba in December '07 (7th - 20th) and the weather was sunny with 5 minute downpours the whole two weeks we were there. Actually, it was rather 'cold' standing under the palapa on the beach during the rain.
Then the sun would come out and the days were perfect with a nice breeze which we love when sunning on the beach. We've been told December is their rainy season but we've been there in December before and never experienced weather like we did this year.
Returned again January '08 (31st to 2/14) and the weather was positively glorious and sunny every day but the winds were very strong and a few days we got sand-blasted on the beach.
Before we left we made reservations for Dec (9-23). Love Aruba....been visiting the island for about 15+ years.