I had one over Cuban Airspace, no traffic call. We are air ambulance so we are allowed into RVSM even though our aircraft are not... the problem then comes when you are transitioning to another country, US RVSM practices (which seperate non-rvsm a/c with 2000 vertically) are not applied to our aircraft any longer. Our altimeters are spotty at best between FL350-FL450 and we can see a split right up to the max non-rvsm difference.
Then you have to choose which indication you want to trust - we chose the wrong one (apparently) at FL410, climbing to FL430 while converging with a Continental 737 that was northbound back to US Airspace. They were level at FL400 and I swear to you I could read the name tags!!! We were at best 400 feet vertically seperated, so always use vigilance especially when abroad and even still when in the US.