I had to do this in Lion which already has a service for this, but here is how to do it. It may vary slightly on (snow)Leopard.
1. launch automator and start with a new service.
2. Change "service receives selected to "files and folders" in "any application". You could choose finder here, but then you won't be able to attach files located on the desktop.
3. under "mail actions" drag "new mail message" to the workflow pane on the right
4. file > save > "give appropriate name" i.e. "Create Email Attachment" or something like it.
that's it.
to test it out, select the file(s) and then from the menu bar select finder > services > and the name you gave to the service. it should create a new email with the selected files attached.
heres a quick screen shot
http://cl.ly/2f0l2Y083g2Q0m2m0W2j