As a Gold, I have been able to get upgraded to a suite several times. I stayed there about a dozen times last year, and was upgraded for 9 out of 12.
In my expereince, best thing to do is call the front desk manager on the morning before check-in with your request. This is when they preassign rooms. An advance request won't do good, as the front desk crew rarely see those notes in the computer.
As a Diamond, you may get the upgrade automatically -- but it can't hurt to call.
Just noticed that you are traveling with a colleague. Depending on your relationship, you may not want a suite. To my knowledge, all suites have King beds. The living section has a couch pull-out bed. If you request a suite, be specific about your bed needs.
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On the road,
In the air,
I enjoy travel
From here to there.
[This message has been edited by ontheroad (edited 12-21-2001).]