Backpack gets what they call a Web services API … I don’t have the slightest idea what this is supposed to be. It’s neither Web services nor RESTful nor anything else I’ve seen — it looks like bad re-invention of SOAP to me.
Update: Be sure to check out the comments.
It’s just XML over HTTP. We didn’t see a need for wrapping that in further complexity. Just like the “REST” API for Flickr. Or 43things. Or even Amazon. Lots of examples of people just doing XML over HTTP without having any aspirations to do a Web Services thing when they’re happy just doing web services. We can get by without capitalization ;)