I disagree a bit in that slate device have never been very popular in mainstream ever. If you want instant on that is not going to be a full PC OS, it will be Windows mobile/Pocket PC OS. Tablet and slate PC's have never garnered much interest except for niche techies.

If you look at the mobile market the majority of mainstream users prefer a touch type keyboard input not a pen input. The majority of mobile devices are laptops with over 113 million sold each year. What has never been created is a pocket version of that popular form factor, a pocket laptop. Now that technology can enable that, it has still eluded the market as most companies have not a clue what maisntream users want.

The UMPC's have failed due to them not having a touch type keyboard and not being small' enough to be jacket pocket size. As I said look at the history of pen input and there is no good mainstream interest. That is why pen input will never be that popular. Due to the size of UMPC's not being able to be carried in a pocket they then compete against all other such devices including laptops. The result is in 2007 only 350K worldwide sold.
Any device that is not pocketable will be compared to laptops and all other non pocketable devices and sorry but a slate has no chance in that realm.

UMPC's run full windows OS and that is not designed nor do people want anything short of a touch type keyboard. Instant on is not what desktop OS provides. There is a huge demand for a pocket laptop with a touch type keyboard that can rull full windows. A battery life of at least comprable to laptops 2-4 hours would still provide strong demand.