It works the same in Colosseum and XD too. Since I do all my training in XD, I have several infected Pokémon boxed in Emerald, who only come out for the purpose of spreading the Pokérus to something I want to train---which then promptly gets sent off to XD so it can never lose the Pokérus.
In short:
FR/LG/Colo/XD: Pokérus effect applies, but it cannot spread or go away
R/S/E: Can spread, but also goes away if the carrier isn't boxed