I was looking at the Arduino stuff for an upcoming project but I've ended up getting some MicroX kits which are Microchip PIC based. The main reason was that even though I've used both Atmel and PIC uC's I've found the MPLAB IDE for the PIC chips a bit easier to use.
Like others have suggested, grab yourself a board, maybe one of the simple one with a couple of I/O on it and some LED's to flash and start playing.
I'm happy to help if you need any specific help
Cheers,
Simon