DW and I are United *G
United posts this on its website

"When traveling on a Star Alliance member-operated flight, eligible Star Alliance Gold members (MileagePlus Premier 1KŪ, Premier Platinum and Premier Gold) may visit designated Star Alliance member lounges. United and US Airways Star Alliance Gold members may access any United Club or US Airways Club only when holding a boarding pass for a same-day international Star Alliance member flight. United Star Alliance Gold members may access other Star Alliance member lounges before any departure on a Star Alliance member airline flight."

So what is an international flight?

In my case I was Flying from JNB to IAD over JFK on a single United airlines ticket .
I had in my possession my boarding cards for both the flight leg I had just taken from JNB on that date and for the connection on United to IAD. It was all one ticket. The Montreal Convention (which is both the law of the land and a part of the contract of carriage) is clear that all segments of such a flight are "international"

1. This Convention applies to all international carriage of persons, baggage or cargo performed by aircraft for reward. .....

2. For the purposes of this Convention, the expression international carriage means any carriage in which, according to the agreement between the parties, the place of departure and the place of destination, whether or not there be a break in the carriage or a transhipment, are situated either within the territories of two States Parties, or within the territory of a single State Party if there is an agreed stopping place within the territory of another State, even if that State is not a State Party. Carriage between two points within the territory of a single State Party without an agreed stopping place within the territory of another State is not international carriage for the purposes of this Convention.

3. Carriage to be performed by several successive carriers is deemed, for the purposes of this Convention, to be one undivided carriage if it has been regarded by the parties as a single operation, whether it has been agreed upon under the form of a single contract or of a series of contracts, and it does not lose its international character merely because one contract or a series of contracts is to be performed entirely within the territory of the same State.http://www.dot.gov/ost/ogc/Montreal1999.pdf

UNITED, like all airlines always insists that the JFK -WAS flight is an international carriage under the Montreal convention.
ADVICE TO INTERNATIONAL PASSENGERS ON CARRIER LIABILITY

"Passengers on a journey involving an ultimate destination or a stop in a country other than the country of departure are advised that international treaties known as the Montreal Convention, or its predecessor, the Warsaw Convention, including its amendments, may apply to the entire journey, including any portion thereof within a country. For such passengers, the treaty, including special contracts of carriage embodied in applicable tariffs, governs and may limit the liability of the Carrier with respect to death or injury to passengers, and for destruction or loss of, or damage to, baggage, and for delay of passengers and baggage." http://www.united.com/web/en-US/content/contract.aspx

I was rather stunned when the lounge person suggested we were not eligible. I pointed out that I had BOTH the same day flight boarding card from JNB and I was boarding a flight that was unquestionably an international flight under the Montreal convention. A supervisor intervened but I wondered if anyone else had this experience.