Here is the URL of a FAQ from a US training school offering IMC training. I just searched google for "IMC Rating". There are other hits, but this one seems to answer your questions:
http://www.ukft.com/faq.html
There is no such thing as IMC in FAA - I wish there was - it's a good halfway house, if all you want to do is fly in instrument wx
Another hit shows that IMC allows you to fly:
Out of sight of the surface
In a control zone on a special VFR clearance with flight visibility of less than 10km but not less than 3km.
During takeoff and landing with flight visibility below cloud of not less than 1800m.
When in IMC outside of controlled airspace notified for the purpose of Schedule 8 to the Air Navigation Order (ANO)*.
Carrying passengers above 300ft above mean sea level (AMSL) in IMC or in flight visibility less than 5km at or below that height, when outside controlled airspace notified for the purpose of Schedule 8.
[ 26 October 2001: Message edited by: sanjosebaz ]