Gentlemen, gentlemen. If concurrency without interrupts or convoluted schedulers is desired, humbly suggest you look at the Propeller chip/boards (at parallax.com). It's 32 bit with 8 cores that run truly in parallel at 80 Mhz. You no longer have to program it in SPIN or assembly, as there's a new Win/Mac/linux C/C++ IDE (coded in C w/ Qt not slow Java). Break up problems into parallel tasks between the 8 cores. All Prop pins are general purpose. On a Prop if you need a UART, you load up UART code into a core, let 'er rip, read and write data via shared memeory. Need VGA out? Dedicate a core, add a few resistors. The chip is $7. There are also Arduino shield compatible Boards, like the ASC+ (with a lot of goodies) at $49. Nice.