Some 2.1/2.2 apps not working I just upgraded my Galaxy gt i5800 to 2.2 froyo from the kies firmware.
Why am I getting "not compatible with this device" from some apps that say they require 2.1 or 2.2? Can anyone please explain.
The "Not compatible with this device" message encompasses more than just the OS version.  The Play Store also checks for other software and hardware components that the app's developer listed as requirements.  These components can range from a minimum screen resolution, to having particular hardware (e.g. Tegra 3 chipset).

You can try searching for a given app on Google Play Store website to get a better idea of the incompatibility.