"Only get, put, and delete requests are allowed" on an XML request... why??

My app on rails 2.3.2 has a standard scaffold for a resource "thing"
I try to edit a "thing" like this:

curl -d '<thing><name>pallone</name></thing>' 'http://0.0.0.0:3000/
things/1' -d '_method=put' -H 'Content-type: application/xml'

and I get the error "Only get, put, and delete requests are allowed"
why? It's driving me crazy...

(My app is accessed by a flex3 frontend that speaks XML, and can only
send get and post requests. It work as expected with html forms, but I
have to use XML !)