Steve Maine's Isomorphism post is really good until the end, where he tries to make a point about Amazon.com's "touted" RESTful web service API.
The REST crowd touts Amazon's human-focused_service_ as a REST example, not the WS API.
Only Amazon themselves claim that their non-SOAP API is RESTful, and I believe it's because they're confused. As well, there's not a marketing term for "simply using HTTP query strings to invoke RPC which returns XML without doing SOAP or XML/RPC". Thus, everyone calls it REST, even though it isn't, necessarily.
So, the reason that <wsa:Action>urn:GetCart</wsa:Action> and http://webservices.amazon.com/onca/xml?Operation=CartGet[...] are isomorphic is because they trivially are. They're both RPC.
Re-creating the same thing in a purely RESTful way would not look isomorphic, depending on how much of HTTP you want to exploit. I'll leave that as an exercise for the reader, for the moment. As well, Joe Gregorio has been developing that idea out.