Mobile phones are an amazing platform to do... well, almost anything. There are some areas where their possession is restricted, though I suspect a motivated party could sneak a stripped down mobile device into nearly anywhere.
Keyboards, on the other hand. Wow. I've seen even airgapped systems have random keyboards right off the pallet slapped onto them. These sit in racks for months or years, then get tossed usually to a recycler, a donation program, stolen, or just thrown into a dumpster. Considering how much tech is in a keyboard, and how much volume it has, you could place nearly anything in there and possibly go ages without catching on.
A scenario that I recently pointed out as a 'thought exercise' was a refitted USB keyboard with a microphone, pinhole camera, and simple keylogger+screenshot engine that contained an intermittent RF/wifi/bluetooth/ultrasonic network. Programmed to dump its payload whenever an individual passed nearby and triggered it remotely.
Such a trojan could sit in a datacenter or conference room for years completely unnoticed. The data it captured transmitted only to the cleaning crew or whatever.
Worse yet, such a device could also pass instructions to the system it was attached to as an actual USB device.
You could fit a lot of horsepower in an innocuous Dell or MS or whatever mass-produced keyboard. Toss it into a top level conference room for corporate espionage, toss it into a data center for more direct trouble, whatever.
Scary thought, and I think part of why I still use the same keyboard I've had since 1997 ;)