Will Android tablets have issues if they are always on? I plan on using an Android tablet in an always-on scenario (a fancy self-coded alarm clock, but this would apply to numerous uses).  Is there any problems with keeping an Android tablet on and charging all the time?  Is this likely to cause screen issues?
OLED screen are very bad for burn in and colour leaking (even worse that plasma screens in alot of cases) en.wikipedia.org/wiki/Screen_burn-in
and so leaving them on the same screen for some time would not be a good idea
You might also find the general life of the device shortened because of the hardware being used all the time, but unless you are putting the device under stress im sure it would be fine.
The battery would likely weaken over time as well due to the constant charging