I've never seen + used in place of %20 in a URL. Decided that since I spent a few minutes thinking on this one to share the references.
RFC 2396:
http://www.ietf.org/rfc/rfc2396.txt, sections 2.2 and 3.4.
I visualize URI = URL + URN. I think of the query
string as a kind of extension to the URI and not part of my HTTP request, the URN located at URL.
A Google of this topic got several hits to the effect that %20 is interchangeable with + in a URL. They all have a similar example URL.
I finally found this:
http://www.w3.org/Addressing/URL/4_URI_Recommentations.html, the section Query strings.
This method was used to make
query URIs easier to pass in systems which did not allow spaces.