Una volta mi era stato chiesto, e non avevo saputo rispondere... Ma ora ho la risposta:
(1) If the person wants a future business relationship with Microsoft in relation to an idea:
https://members.microsoft.com/omc/Welcome.aspx
There's important information about using the portal here:
https://members.microsoft.com/omc/faq.aspx
(2) If the person simply wants to provide an idea for Microsoft to do with as Microsoft wants:
http://www.microsoft.com/globaldev/outreach/ideas/ideaSubmit.mspx
(3) There's also a page with advice for start-up companies who want to work on their idea themselves.
http://www.microsoft.com/business/startups/default.aspx