This may be a dumb question, but is the Ultegra FC-6500 53/39 (9 speed) crankset compatible with an Ultegra 10 sp cassette?
I've been told that it is not. Mixing 9 and 10 speed components (Chainrings/cogs) can cause shifting problems.

I don't think there would be an issue..... I have a beer riding on the answer..

Thanks in advance

Generally cranks don't care much how many cogs are in back of them and the spacing difference between the chainrings of 9 and 10-speed cranks is minimal. The concensus is that 9-speed cranks work fine with a 10-speed cassette and chain.