Don't forget that mileage based upgrades are possible in the US program.
I'd refer people to the handbook for more info. But to answer the most commonly asked questions:
Travel must originate in the US.
You can only upgrade from unrestricted fares, which means fares with no penalty for changes or cancellation. Generally B2 is the cheapest unrestricted WT fare, and T2 is the cheapest unrestricted WT+ fare.
For US or Canada to Europe:
For routes with WT+:
25K for WT to WT+
35K for WT+ to CW
For routes without WT+:
40K for WT to CW
Club World to First or First to Concorde:
40K
Refer to the handbook for other destinations.