I just wanted to share this. I am traveling to India this month and will be staying in a Royal Meridien hotel. I called Starwood Preferred Guest to activate a Gold account they offered me last year. I mentioned that I am platinum at Marriott and the agent told me that if I send in my Marriott credentials, starwood would likely match my platinum status. I faxed a copy of my card and a printout of my account info and within hours I received an email stating that they would grant me SPG platinum status for the rest of the year! Now I can get upgrades and use the executive lounge in the Meridien. Its worth checking into if you have to stay at a non-Marriott hotel. I am going to try the same thing with Hilton - will let you know if it works.
It does work at Hilton. I asked earlier this year and was upgraded to Diamond. Just had to fax my last three months of Marriott activity to them.
Make sure you check out all the ways to get extra points on their website! 500 pts for every online booking with your AmEx, 1000 pts every stay with My Way Preferences, 50% pts for Diamond status, 50% pts for Points and Points preference (or you can choose miles). I've been there since January and already have enough points for 5 nights in Sorrento and 4 nights in Florence, with points left over!