Hi
It is not possible to retrieve serial number with SCE. The inventory feature is limited to the standard attributes (around 30) and can't be extended. In SCCM2007 you are able to extend the inventory but not in SCE.
The same thing goes for product serials. I don't know if you are able to retrieve that in SCCM2007, I think it might be scrambled somehow but I haven't tried to get it out.
SCE is a great product but there are some "hidden" limitations that can be really frustrating.