That's a huge improvement, really well done daverave. Rust would be difficult, I think your color blend is nice now it looks so much more realistic. The thing with texture painting is to get as much color variation in the maps as possible, I like how you can zoom in at pixel level in CS5, if you do it on a photo of skin you'll see that you have all colors under the sky in there.
For the shadows now you'll need more shadow rays to get the blur soft, it will increase your render times like mad when you take this up. This is also why spot lights are so commonly used, they have excellent depth map shadows but it doesn't work with final gather and dome lighting. I also think that a three point lighting scheme with depth map shadows will make that texture look better and should be good enough. It's something that's a lot faster to test render as well and so much more flexible.