I asked myself the same, knowing the 990 would be out soon, before I bought my 910. I'd have to say NO. The cost doesn't warrant the changes in the new version. The camera is better, and resolution for pics, and the backlight on the keypad is appealing. Other than that, there isn't a HUGE fuss over changes. I'm still having fun on my 910, after almost a year of having it, and haven't felt the temptation to want to spend that much money for the upgrade.
It depends if you have Cingular then it is a disadvantage. For new coverage, Cingular is building 850 GSM towers. P990 doesn't have the 850. I work with someone who has it. he loves it, but gets somewhat weaker signal than my P910A.