Dual-core doesn't sound all that crazy.
Texas Instruments has recently announced the OMAP4 series of SOCs that include the
ARM Cortex-A9 MPCore. If Apple can build a system around it and maintain good battery life, they most certainly will. And why not, they'll charge and arm, leg, and kidney for it anyway.
Same new core tech as the Nvidia Tegra 2, specifically the Tegra 250 chip (Tegra 2 is a SOC).
As the Cortex-A9 is simply an update to the Cortex line, the entirely Cortex line using the ARMv7 instruction set, with A9 just employing some new tricks compared to the A8 line... I imagine it wouldn't take a great deal of work making a new OS version work just as well on both the A8 series and the A9 series. Considering the iPhone 3Gs employed a Cortex-A8 chip, as do the Moto Droid and Palm Pre... Apple might want to up the ante. And well, they need to if they want to stay relevant in terms of hardware potential. Like I said, Tegra is gaining in popularity thanks to the reveal of Tegra 2 (and the Tegra line wasn't exactly hurting, but wasn't seeing a ton of business, unlike TI with the OMAP SOCs). Tegra 2 using a multicore Cortex A9, Qualcomm showing off the upcoming Snapdragon chips >1.0ghz and (and multicore? regardless, based on ARMv7, so Qualcomm going from Cortex A8-based tech to A9 isn't exactly far-fetched), and TI announcing the OMAP4 series... basically unless Apple is retarded, they'll have dual-core.
Then again, Apple is retarded. It's easy to get that way when you can whip fanboys up into a foaming-mouth frenzy as if your new product was touched some Deity.
And again, with Apple's methods in mind, the tablet will likely feature one of these Cortex-A9 chips, and maybe just include a faster Cortex-A8 in the next iPhone, or a single-core A9.