Vallejo makes a line of air acrylics. I have not tried them, but many miniatures painters swear by them for brush painting, especially the "metals" colors. I do have extensive experience with brush painting Vallejo's model color line for painting miniatures. While I like their model color line they can be a bit uneven. One color may dry flat while another dries more satin. One color may provide good single coat coverage while another may require multiple coats. One smooth and another grainy.
Norm, In my experience Vallejo's model color is no less durable when applied on metal than most other acrylic paints. It is also no more durable then most acrylic paints either when applied on metal, and that is the rub. If you can do it properly, put down an appropriate primer before painting and then hit it with varnish after it has cured you have a much better chance of having it hold up. However if you are using acrylics for touch up you might not want to risk doing the priming and varnish sealing steps for fear of messing up the model.
I think Walt that even if Rich is looking for brush paints, those same paints usefulness as air brush paints is important. If you find a color that is absolutely perfect for one application method it would be nice to know you can also use it with the other application method.