Costco did carry the original Transformer, only not until a few months after its release. I called them a few weeks back and they didn't have any plans to carry the Prime, but of course they don't really make long-term plans like that. They often wait until they can get certain terms, and then sell things--which is why they don't always sell things at the very beginning of their lifecycles. The Xoom is an exception--they had it on launch day.
I imagine they'll have the Prime eventually. They've sold pretty much every Honeycomb tablet at one time or another.