FBO extension?

i coded a little app using FBO! A few days ago i recognized that some older graphiccards don’t support the FBO, but it works anyway on such systems without any problems testet it on 5 pcs without FBO-support!

So is it bad to run it on such sys?
What could happen in worst case?

On what OS are you running your application? Unix one?
In this case, it probably works because mesa3d drivers are installed.
Even if the framebuffer objects are not supported by the hardware, mesa drivers will run it in software mode.

So, to answer your last question, your program will certainly be slower.

Running under WinXp + Vista.
So, under Windows it runs on a INTEL 910GML-onboard chipset (laptop) which doesn’t support FBO, but it runs anyway.
Who takes care of the FBO if there is no hardware- and normaly no sofware-support under win?
Must there be any special driver installed?
PS: The application is for commercial use, so i should be able to list prerequisites.

FBOs are extensions, so if your code work, then it is supported on your card. The thing, certainly, is that everything about FBOs fallback into software.