Marc, First SPServices Rocks!
That aside, I am curious if you ever considered instead of valuepairs, to just use Key: Value from JSON?
BACKGROUND: I have been incorporating SPServices into John Papa's Single Page Application (http://www.johnpapa.net/tag/spa/) (similar to http://boilerplatejs.org/) with a single file I call dataconnection that uses SPServices as the core for CRUD operations on lists. I am using your XMLtoJSON to get the data into the Application which works great! I made a few modification such as auto detection of the column type using RegEx in order to have a more palatable json object without having to specify a mapping in order to use a single dataconnection library for any list the app uses (e.g., lookup becomes {lookupId: thisLookup[0], lookupValue: thisLookup[1], columnType: "Lookup"}) (although I did keep a way that I can easily override the mapping from a config file if I want to keep things slim but key was to be able to have a default route that would add any list data to the app with as few steps as possible)
Right now in order to do an update on a list item from my application's objects, I have to parse the javascript object into the valuepair format (which isn't a big deal but a little more code); however, if this could be done as a reverse of the XMLToJSON (which I am currently working on the mod) then it could be a little more transparent although granted a little more bloated.
Just curious if you have explored this, had this planned this in an update, think that I may be trying something that isn't feasible or any other words of wisdom.