No. In Summer ‘05, custom formulas can only reference standard and custom fields within the same object. They cannot reference fields in their “child” objects (i.e., related lists).   

We plan to support sums & counts in the future. If you want to add your voice to make this enhancement a priority, please file a feature request case f or the following feature:

- Summary Fields – ability to count or sum fields in children objects (i.e., related lists)