That is really cute. I hadn't noticed it before. Although, I did notice it in chick flick (season 3), when she found Piper in the shower. One of the best scenes of charmed I personally think, maybe that's why I noticed it.
Anyway, to get back on topic, I've always had this idea in my mind of getting Prue and Andy back together. Like he'd return as a whitelighter and he would be able to save Prue in All Hell Breaks Loose. They were just so great together.