For what it's worth, when the G-8 summit was held in the Calgary area in 2002, a digital Smartzone 400mhz system was brought in and erected specifically for the event, using portable trailer-mounted repeaters, etc., and was 'piggybacked' onto our local Smartzone system so the local police could be involved.
The 400mhz portable part of the system is, as I've mentioned before, rumored to be in the Toronto-Ottawa-Montreal area these days. Check the 420mhz part of the band..
EDIT: And no, neither the secret service nor anyone else can just use their radios "anywhere in the world". What may be reserved government band in one region (country) may be public airwaves in another. This is kind of a lesser example than the USSS, but I remember about a decade ago when the police service here was on the 462mhz band, and a number of our American friends came up during a rather large event (think horses). Guess what invaded on one of the busiest channels in the police system? The same kind of thing could happen anywhere with anything. My fire radio is on a 413/418 split. I couldn't just take it into another city - let alone another country - because I would be walking on other transmissions.