That's a horrendously complex question to answer, and really one would have to know the specific situation to venture an opinion.
and you should consult a lawyer.
Here are a couple of guidelines and caveats anyway. A trademark owner can always take a domain owner to arbitration at WIPO. This only works for second level names.
Registrars often stipulate that domain disputes will be subject to the laws of their own country. This may not be legal in fact, but the bottom line is that they'll take you name away from you anyway. There are also some unique country exemptions, for example you can use the word "olympic" in parts of Greece.
There also some practical consequences to using domains with trademarks in them as well - for example, you can lose your hosting or your sponsor accounts.
To be quite honest, unless you're a stoppy bastard, and fairly clued up, then you should leave trademarks alone.