The Automator workflow I used in Tiger no longer works in Leopard. If you use more than one "Move Finder Items" or it is not at the end of the workflow then you will notice an error if no files are passed to it.
I had three "Move Finder Items" with filters before each.
In Leopard, the Move Finder Items errors. It would be nice if they added an option for it to error or not, but they did not and it broke many peoples workflows.
My solution is to use Run Shell Script in place of Move Finder Items.