require "xmlrpc/client";

server  ="";, "/cgi-bin/mt/mt-xmlrpc.cgi";)
content = { :title => ‘Testpost using ruby and xml-rpc', :description => ‘hello world' }
result  ="metaWeblog.newPost";, 5, "USER";, "PASSWORD";, content, true)

works like a charm :)

Just to make some things clear:
/cgi-bin/mt/mt-xmlrpc.cgi   <— movable type xmlrpc file
5                                     <— my blog id (I could see it in my admin interfaces URL)

Good sources to look at:

A good piece of source concerning the upload of pictures using "newMediaObject";: