Behind the Velvet Rope
The AMEX Platinum personal credit card sign-up bonus has been upped from 50K to 75K. While this is an improved offer, I'd personally recommend holding off on signing up for the card because I'm certain AMEX will up the bonus to 100K at some point this year, just like they did last year.
One reason to ignore the advice above, though, would be if you could immediately reap the rewards from the statuses or card benefits the AMEX platinum offers, such as SPG, Marriott, and Hilton gold status or the little talked about 3rd or 4th night free benefit.
For example, if you'll be in Tokyo soon, the benefits you get with the AMEX platinum may be worth signing up for the card now. The Park Hyatt Tokyo, of Lost in Translation fame, currently offers a complimentary 3rd night when booking with an AMEX Platinum card.