Ars Technica's Chris Foresman has a terrific interview up with Star Trek production artists Michael Okuda, Denise Okuda, and Doug Drexler about how they conceptualized iPad-like devices for TV and movies... some 23 years ago. Their device was known as the PADD (Personal Access Display Devices):
Drexler said that to him, the iPad is "eerily similar" to the PADDs used in Star Trek. "We always felt that the classic Okuda T-bar graphic was malleable, and that you could stretch and rearrange it to suit your task, just like the iPad," he said. "The PADD never had a keyboard as part of its casing, just like the iPad. Its geometry is almost exactly the same—the corner radius, the thickness, and overall rectangular shape."
"It's uncanny to have a PADD that really works," Drexler said, unlike the non-functional props made for the TV series and later films. "The iPad is the true Star Trek dream," Drexler told Ars.