Same here, I've been able to go back and forth using layered .psd files without issues.

A trick to use CS4 Brushes in CS3: in CS4, paint each brush on its own layer. Open that .psd file in CS3, and then define a brush preset for each layer, one at a time. It can be tedious, but is a workaround. I just did this between CS4 and Photoshop 6 for a small brush set I made in CS4, worked fine.