I suppose I could make this into a library; using the string library and crap load of substrings
Actually, this has already been done: the Webduino library is great for building apps that need more of a "standard" GET/POST interface (if I do say so, myself, as a contributor who added most of the GET/POST support to Ben's initial efforts ::))
It's got a pretty hefty memory footprint, though, so a usable (i.e., easy to understand and change) example of doing GET for an extremely limited set of URL-encoded parameters would be a boon to quite a few people who don't need the rich set of features included in Webduino. I'm actually looking at such a situation, myself, at the moment: I'm working on a remote sensor that will log its readings to a website hosted by godaddy.com, and I'm thinking about adding the ability to change the hard-coded DNS address without needing to drive out to the site and reload the firmware if godaddy moves their DNS server. It looks like replacing the simple server in the Arduino that I put in for debugging the system and checking its status remotely with Webduino may run me out of memory. So I may want to use your example for adding the one parameter (DNS IP address) I need.
POST is quite different from GET, btw: POST parameters show up in the body of the request, not encoded in the URL. That's harder for the Arduino to interpret, and requires additional code.