Arduino platform is great in its simplicity and tools out of box. When you need to interface something to Arduino board you can use shields or prototype on breadboard. Some hobbyists just go different way – they use AVR chip with Arduino bootloader and make custom PCB with desired features. Check out new Arduino development board that seems really attractive.
It is based on ATMega1284 microcontroller featuring 16K of RAM, 4K EEPROM and 128K flash. There are plenty popular peripherals already on board. These include Real Time Clock DS1307 with backup lithium battery. SD card socket, RS232 driver, FTDI FT232RL USB driver, JTAG header. And more handy stuff.