"Valve Sabotaging 360 version of Portal" is paranoia at its finest. In all likelihood, it uses the exact same Source Engine port used for L4D2 rather than a port that was deliberately downgraded by Valve software engineers. They have plenty on their plates as is without making more unnecessary work for themselves.
What's more probable is that Valve took the time to utilize the resources of the PS3 to create an optimized port of Source Engine for the platform. It's not a conspiracy.
As for the inevitable PC vs. console port comparison, the PS3 port really didn't lose much in the translation as any PS3 owner (who got a free copy of the PC game) will tell you. I've been playing it on Mac, PC and PS3 and about the only thing missing from the PS3 port is the ability to upload game saves to Cloud. It does however, download Cloud game save data.