Here's the schematic for the ethernet board - you should be able to open this up in a new window and enlarge it to see the detail. If you want to receive this by email in EagleCAD format please email me at ken dot boak at gmail dot com or get in touch via Twitter @Monsonite.
This is about as simple as it gets, the ATmega328 with 16MHz clock circuit. All of the '328 I/O lines are brought out as per the Arduino. The power supply consists of a dc input feeding a 5V and 3V3 voltage regulator. The ENC28J60 has a 25MHz crystal and connects to the '328 via the SPI bus. A 74AHC125 acts as a level converter for the /INT and MOSI lines from the ANC28J60 to the '328. The ethernet connection is simplified by using a Magjack - an RJ45 connector with all the magnetics integrated into the connector.
The prototype was breadboarded and I used a FTDI cable to program it and supply power. It is currently supplying regular temperature readings up to Pachube