Vacation and a nasty case of the stomach flu have put me behind in posting but I’m starting to catch up. I wanted to point out that Chris Roehrich of Microsoft has a post up on Developing for Dynamics GP. Chris takes a look at using web services to get quantity amounts like Quantity on Hand for Inventory Items.
Retrieve Quantity on Hand using Web Services for Dynamics GP 10
Development & IntegrationGP 10
Archived post from 2008. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.