Steve Ballmer’s keynote at the Consumer Electronics Show in Las Vegas Wednesday night ran for more than an hour and a quarter.

But what got the most attention from the press gathered to watch it were the three minutes Microsoft’s (MSFT) CEO spent talking about his answer to the tablet computer Apple (AAPL) is expected to unveil in less than three weeks.

Ballmer displayed three versions of what he called “slate PCs” running Windows 7 — one from Pegatron Corp., another from Arcos, and a prototype from Hewlett Packard (HPQ) that Ballmer says is coming “later this year.”

In a subtle dig at his competitors in Cupertino, he demoed the HP Slate with an Amazon (AMZN) Kindle version of Stephenie Meyer’s “Twilight.” The book’s cover shows a pair of hands cradling an apple.

This is not Microsoft’s first venture into tablet computing. Ballmer did not mention his flagging Ultra-Mobile PC project, which got so much attention three years ago, or the Microsoft Courier, a two-sided prototype tablet that has been gathering trade-press buzz since September.

You can watch Ballmer’s full keynote here. The slate portion begins at the 49:30 mark.

Below the fold: a hand-held video of the three minutes devoted to the slate PCs, courtesy of TechSanityCheck, as well as the 18-second promotional video that HP released Wednesday night.

http://www.youtube.com/watch?v=IIIjTDnX2Y0

Little is known about the HP Slate except that it has a multi-touch screen, instant-on capability and appears to be about 1.4 cm thick. According to Electronista, it runs a Qualcomm (QCOM) Snapdragon processor, has all-day battery life and relies on flash memory.

[Follow Philip Elmer-DeWitt on Twitter @philiped]