Arduino Nano is a surface mount breadboard embedded version with integrated USB.
It is small, complete, and breadboard friendly. It has everything that Diecimila has (electrically) with more analog input pins and onboard +5V AREF jumper.
Physically, it is missing power jack and power select jumper.
Since the Nano is automatically sense and switch to the higher potential source of power, there is no need for the power select jumper.
Microcontroller: Atmel ATmega168 or ATmega328
Operating Voltage (logic level): 5 V
Input Voltage (recommended): 7-12 V
Input Voltage (limits): 6-20 V
Digital I/O Pins: 14 (of which 6 provide PWM output)
Analog Input Pins: 8
DC Current per I/O Pin: 40 mA
Flash Memory: 16 KB (ATmega168) or 32 KB (ATmega328) of which 2 KB used by bootloader
SRAM: 1 KB (ATmega168) or 2 KB (ATmega328)
EEPROM: 512 bytes (ATmega168) or 1 KB (ATmega328)
Clock Speed: 16 MHz
Dimensions: 0.73" x 1.70"