The original Portal on PS3 is not. At the time, Valve gaffer Gabe Newell hated the PlayStation 3. Now he loves it. Business, etc.
With Portal 2 now readily available worldwide, the developer has confirmed that the original game was a bigger success than many first assumed. It's actually sold nearly four million copies. And that's not including sales via Valve's Steam service. Which would probably double the number.
A bulk of those sales have come via its inclusion in The Orange Box, which did release on the PlayStation 3 — albeit half-heartedly. We assume given the poor performance of that PlayStation 3 port, a bulk of the sales are for the XBOX 360 and PC versions. Still, we imagine a big chunk of Portal 2's sales will be on Sony's platform.