enamel clear over acrylic paint?

I spray Testor's Dull-cote (which is a lacquer) over Tamiya Acrylics and haven't ran into any problems at all.

great, thanks. Just picked some up, thought I would try it after final assembly. (Before the windows, haha)

Absolutely .... unless you want that frosted glass look.

Is it okay to paint a model using acrylics for it's easy of use and simple cover it with a coat of clear enamel for protection?

In short ... absolutely.

For a matt finish I use dull-cote as desribed in the previous post.

If I want a gloss-cote (and this is very rare for what I build) I use Future, it works very well. But a few other modellers use enamel or lacquer gloss-cotes with good success.