According to the infallible Wikipedia, the first device in this form factor marketed in the United states used the word pad.
1989: The first commercially available tablet-type portable computer was the GRiDPad from GRiD SystemsThen you see the name appear from IBM a few years later.
1992: GO Corporation shipped the PenPoint OS for general availability and IBM announced IBM 2125 pen computer (the first IBM model named "ThinkPad") in April.Of course, the design of the commercial versions of the IBM ThinkPad differed significantly from the prototype's keyboard-less tablet design. The point is that the first tablet-like compters were named pads. The name has fit since the form factor was created. It still fits best today. Tablet? A failed movement that conjures up images of Bill Gates explaining year after year why tablets haven't broken through commercially. Tablet is a terrible brand name and doesn't work that well to describe the form factor generally, in my opinion.
Apple doesn't appear on the pad computing scene until it begins selling MessagePads on the Newton platform in 1993. Yes, the MessagePad was a PDA, but :
...although it ultimately became a PDA, its original concept (which called for a larger screen and greater sketching capabilities) resembled the hardware of a Tablet PC.The word pad was the most obvious and most natural way to describe the device from the earliest days. It still works best. The word will become synonymous with tablet computers and eventually replace it in the consumer vernacular. No consumer 'on the street' has ever desired a tablet pc. They will need a pad computer. That's Apple's magic. Given Apple's near perfect user experience development, the word pad is sure to have strong appeal in the marketplace. Competitors should understand this and continue to use the word - not shy away. HP Slate? Microsoft Slate Tablet? To my ear, both sound cold and have nothing but negative connotations. A pad is comfortable to hold in your hand and easily take on the go. HP LifePad, Microsoft Padsoft, MicroPad or WinPad make so much more sense to me. HP and Microsoft - you can have those names, no charge. Why fight it? Embrace it - Apple is giving you a gift here.
Apple didn't coin the word pad and Apple is just one maker of pad computers. Not only that, Fujitsu has been marketing an iPad mobile tablet computer for years (and owns the trademark). As opposed to the iPod, Apple is breaking no new branding ground with iPad. It would have been silly and probably infringing on a trademark for Microsoft to call their MP3 player a ZunePod or MicroPod. However, I can't see any reason they wouldn't call their pad computer a WinPad (a name that has ironically made the rounds in years past for both Microsoft and Apple's FingerWorks team, yet is not trademarked in the U.S. today).
Further, if there are 150k applications built for the iPhone, there will be 1M applications for the iPad in a year or two. They will embrace the word pad as well. Pad applications will be some of the most useful and creative software ever created. And companies will make pad application versions for the iPad as well as all the other pad computers on the market. If I built an app called PadBand, I wouldn't likely also brand it TabletBand or SlateBand. The device isn't what will create the economy, it will be the apps. The word pad will be everywhere. Think about the pad possibilities with a device with such a large HD screen, yet still mobile: mobile live video (get ready for padcasting), pad games, pad personals (mobile!), pad tag, mobile MMO's with pad computers out in the wild.
Bottom line, embrace the word pad, people - it's coming. I invite professional and amateur marketing gurus to comment below on the industry's use of the word pad vs. tablet vs. something else.