we need to be very clear on the terms when talking about UX matters.
Pencil is a wireframing/flowcharting tool (which I know very well and often use instead of Balsamiq).
Fluid UI is more of a "low-fi prototype demo" service.
The two are not interchangeable nor synonimous.
@Nic264 I used to work on the core apps project and still have the Balsamiq credentials stored somewhere. The tool is pretty slick (though a little slow at times) and allows for both wireframing and live-testing.