This specification defines a new HTTP 1.1  method PATCH that is
used to apply partial modifications to a HTTP resource. A new method
is necessary to improve interoperability and prevent errors. The PUT
method is already defined to overwrite a resource with a complete new
body, and MUST NOT be reused to do partial changes. Otherwise,
proxies and caches and even clients and servers may get confused as
to the result of the operation.
This is a revival of an expired Internet Draft RFC; it will be very interesting to see how much support this gets. Personally, I’m still undecided; I understand the theoretical value, but I wonder whether the cost of introducing a new verb isn’t prohibitive (which means that PUT or POST would have to be used).