Room acoustics question...

It depends on a couple of things. First, how far down is the ceiling suspended? If it's less than 16-inches, then any reflected sound in the cavity above is amplified which will be a problem. Second, what are the tiles made of? If they are thin cheap fiberboard, then they will transmit sound above (resulting in the same problem as above) plus reflect sound back down in the room. Third, what's the ceiling height? Less than 9-feet could be a problem with early reflections. However, if your ceiling is suspended 16 to 18-inches, the tiles have decent sound absorption and is 9-feet or more above the floor then you have an advantage over a hard, flat untreated ceiling.
The best way to treat a suspended ceiling is to roll insulation (without foil scrim or face) above it. The thicker the insulation, the better. BTW, structural limitations take priority. Also, if there are flourescent lights with flat acrylic lenses, you should consider replacing them with parabolic lensed troffers. Much better sound diffraction.

Ozfly: Acoustic tiles absorb sound at a very un-even rate. What i mean by that is that treble and upper mids are damped whereas mids and bass are left alone. This can skew the tonal balance, making it harder to achieve a natural and even tonal balance. This is not just a problem with acoustic tiles, but a lot of what passes for as "acoustic treatments". Yes, i am talking about the mass majority of brand name stuff from some very "respected" acoustic based manufacturers.

Suspended ceilings can do the same thing as the tiles, but also contribute to rattles, resonances and random oscillaton. They are a true nightmare to say the least.

Combining the two can leave you with nothing less than a mega-dollar system in a room that royally sucks. I'm sure that Rives can fill you in on this better than i can, so for your personal edification regarding room acoustics and musical enjoyment, heeeeeeeeere's Rives Audio : ) Sean
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PS... This was covered pretty recently and should be in the Agon archives

Sorry, please everyone, correct me if I'm wrong, but I thought suspended ceilings with acoustical tiles were good things. They might have a downside if you already have a heavily carpeted and furnished room (i.e., dead sounding with lots of mid/high frequency absorption), otherwise ...

Buy electrostatic, planar, or ribbon, line-source speakers -- they don't disperse sound in the vertical plane, so that should render the composition of your ceiling a moot point... I use these types of speakers in my room with a suspended ceiling and have no problem at all achieving excellent sound.