Windows 7 as an operating system should be able to be tailored to low power devices with little effort. Currently, from my current tests, a formerly very high end (Qosmio G40 with full HD) notebook can sleep and wake with approximately the same performance as iOS 4.2 on iPad.
Additionally, that same notebook can go 5 days on battery in sleep mode and respond to wake on lan events signifying that the ethernet mac and minimum hardware is still functional. Windows 7 on top of that can run for 2-3 hours while watching a film or playing games.
Now, to speculate. If you were to use an iPad level processor meaning something similar to the A5, had a much lower power screen, added support for all the normal bells and whistles regarding power management in a portable device (hardware and driver support, not OS kernel revamp), standby for the system would be the same as the iPad since standby puts the machine itself into a power off state and only the event generating controllers are functioning during that time. In a powered on functioning state, the same technology which makes speedstep function would allow the tablet to run 5-10 hours.
What is however needed to make Windows 7 function properly for a device like this is :
a) Removal of all non-tablet services. Services for supporting desktop tech should be tossed. Services for supporting legacy emulation should be dumped. Services for supporting WMI should be dumped. Keep it bare. This can be done by disabling services in the control panel.
b) Removal of the classical windows desktop shell. It's not designed well for this, it should only be visible and running when the device is docked, charging and has a keyboard and mouse connected.
c) A tablet desktop instead. This would likely be a modified version of the Windows Phone 7 environment which functions fine already on Windows 7. It's just a .NET app after all.
So, I'd really like to know what it is about Windows 7 which isn't suitable for a tablet. I'm just a lowly operating system system level developer, so these complex topics like what you've mentioned confuse me. I think it's best to leave it up to the experts like yourself. I'll check back for a response, I need to get back to developing the boot loader and real time task scheduler for a new linux device I'm working on.