Best platform for learning embedded programming?

Myself I've worked in embedded programming for 9 years now and have experience on TI MSP430, Atmel AVR (a couple of flavours) and will be using an ARM soon.

My suggestion is to pickup something that has some extra features in the processor like ethernet controller and CAN controller, even get two or three if you can. Embedded devices are nice to work with, but once they can talk to other similar devices via CAN or get onto a network, they can become much more fun to play with.

Your question sort of has been answered in this question.

To add to that, the embedded processor industry is very segmented, it doesn't have a major player like Intel/x86 is for the "desktop" processor industry. The ARM processor does have a large share, so does MIPS I believe, and there are many smaller more specific microcontroller like chips available (like the MSP430 etc from TI).

As for documentation, I do embedded development for a day job, and the documentation we have access to (as software developers) is rather sparse. Your best bet is to use the documentation available on the processor manufacturers site.

I would suggest Microchip's PIC18F series. I just started developing for them with the RealICE in-circuit emulator, but the pickit2 is a decent debugger for the price. You could say this for the AVR's also, but there is a large following for the device all over the web. I was able to have a - buggy, yet functional - embedded USB device running within days due to all the PIC related chatter.

The only thing I don't like about the PICs is that a lot of the sample code is VERY entwined into the demo boards. That can make it hard to tear out sections that you need and still have an application that will build and run for your application.