Sparkfun has a number of good sub $50 development boards. Also go to TI for the Stellaris family in the $50 to $100, also the Hawkboard which for now I would recommend before the Beagleboard for what you may want/need$\$$50 development boards. Also go to [TI][2] for the Stellaris family in the $\$$50 to learn, also TI has the MSP430 family and I would get an EZ430 and a three pack of the add-in boards for $10$\$$100, also the [Hawkboard][3] which for now I would recommend before the [Beagleboard][4] for what you may want/need to learn, also [TI][5] has the MSP430 family and I would get an EZ430 and a three pack of the add-in boards for $\$$10. At Sparkfun get a Lillypad with the FTDI USB serial/power board, the Lillypad is pretty much the same as the Arduino pro mini but you need to solder for the pro mini. I am not a fan of the PIC family but you may want to get something there as a history lesson, same goes for the 8051, both families are still popular and in use, just not very efficient and have been passed over by other architectures. Absolutely learn ARM and thumb, maybe MIPS (which is a pic-32, not to be confused with the older original PIC architecture). The ARMmite Pro is a good entry level ARM board, although the Stellaris may be as well.
What you want to learn here is assembler for various platforms. C. C and assembler interaction. Different tools GCC and non-GCC. How to read a datasheet/programmers reference (and realize they all have some errors or can be misleading, never trust them, the hardware wins over documents) and how to read or use a schematic. These are not complicated schematics normally. A number of the boards are good for interfacing in projects meaning they dont have garbage on the board in the way, just direct access to the I/O pins. But that is not the best for learning. Something like a Stellaris board which is painful for projects has lots of fun stuff on board for learning embedded and learning to borrow/use drivers or write your own from datasheets. The Atmel AVR butterfly is a good board too if still available, may need to solder on your own serial port to program it or just jam some wires in the holes. What it gives you is a few peripherals you can learn to program.
Even if you end up doing embedded work that involves writing applications using SDK or API calls on linux or an RTOS (never touching hardware or reading datasheets), the above knowledge will still set you ahead of the rest.