Electronic – Micro-Controller Operating System with GUI

No. The price of a microcontroller running the .NET framework will always be greater than the price for a microcontroller with equivalent performance (or equivalent to the price of a micro with greater performance) which is programmed in assembly, C, or even using the Arduino tools.

When buying a microcontroller for the .NET framework, you pay for small transistors, you pay for more transistors, you pay for complexity, you probably pay for licensing a core developed somewhere else, and, since you're paying for all that, most manufacturers imagine that you'll want to pay for higher pin-count packages since your die budget supports them. If you did this in assembly or C, you could get equivalent performance and pay a lot less.

See also the following questions:

• Arduino / Processing versus .NET Micro Framework? Best language? Best hardware?
• Microcontroller with Java API
• What are the .NET Micro Framework ready systems available?
• Is C++ suitable for embedded systems?

The first question in the list is very applicable to this question, it just discusses the suitability of the .NET micro framework as opposed to the pricing and programming which we're discussing here.

If you still want to use .NET, you think you can get acceptable performance out and you're going to be buying a significant number of parts (at least 1k, probably easier at 10k), you can do the same thing that the vendors of the FEZ parts do: Have it loaded at the factory. It costs somewhere in the region of a quarter apiece to program large files like the one you'll be working with.

You seem to be looking for an MCU that has a power failover built in as a peripheral of sorts. There may be a rare MCU available that has this, but from my memory there aren't many in a general way. For instance, I know that Atmel CortexM3s have the ability to run their internal RTC off of battery power. However, that is about as much that you can do.

Instead of looking for a single chip solution, it may be easier to look at designing a low power system out of a few different pieces, which would be easier to do.

For a microcontroller, pick something that's specifically designed to be low power. Having fine grained power control on the peripherals and the availability of low power sleep states is a plus. I've seen TI's MSP430 line qualify well in those parameters. Olin, in his comment, suggests the PIC16F1xxx series. Neither is an 8051 based solution, though. There may well be 8051 based controllers which qualify. I don't know how amenable the 8051 core is to low power states, though to be fair a lot of these other cores also do most of their sleeping by slowing down system clocks and turning off peripherals, which doesn't really need a specialized core.

In order to switchover power, you could use an IC designed for doing that, depending on the sort of battery and topology you have in mind. For a non-rechargeable coin battery sort of thing, you could look at one of the many RTC and Watchdog and Reset controller ICs, a large number of which include some way to switch over to a battery. For a rechargeable battery, assuming its voltage to high enough to be close to the supply voltage you intend to use with mains power, you're better off using a battery tied bus topology. This means that the battery is always connected to the circuit. Imagine the battery and mcu being connected in parallel. When there is mains power, the battery is charged also (using appropriate charging circuits depending on the chemistry of the battery) and when its absent, the current through the battery reverses and powers the circuit.