Mandrake7062: I suppose there is a few ways to do that but the method I use is:

1. Bake the object at the same size as the objects texture map.

2. Open the objects texture in your paint software. Create a new layer and fill it with your desired dirt colour.

3. Open the Ambient Occlusion bake in the paint software.

4. With the object texture selected, use the ambient occlusion as a mask from image on the dirt layer - change opacity to suit your requirements (this works much better than Cinemas' straight AO).

Can't be more detailed than that because I use Paint Shop Pro and whatever you use may differ to get exactly the same end result.

Have fun :-)