You can get away with a lot in specific cases of trademarks. For example, Cadbury have been able to exert some protection of their usage of purple, but only for chocolate wrappers - not for everything. Similarly here, I can see how the IOC or LOCOG or whoever can exert control over "London 2012" which has a huge association with their brand. Generally it comes down to a fuzzy test of "would it be misleading", and for most people "London 2012 Hotel" probably would be if the hotel wasn't really associated with the event.