eagle: As Cry Havok said, this is only possible with a "transparent firewall" setup.
Searc the forum and the docs for info on how to set that up: http://doc.pfsense.org/index.php/Tutorials
As long as pfSense itself actually routes traffic it's not possible what you're trying to achieve.
But what i'm thinking: why are you trying to hide the pfSense IP?
I mean as it is right now your users can type "ipconfig" on the cmd and they will see the default gateway which is the pfSense IP, so no point in what you're proposing.