Funnel IO is a new board that is being sold through Sparkfun. This board has some pretty promising features including an XBee socket and a Lithium Polymer battery charging circuit. The board is set up so that you can program it wirelessly through the Xbee's serial connection to the AVR. It is designed to be used with the Funnel toolkit which allows you to interface to physical components such as sensors and switches with programming languages such as Actionscript 3, Processing, and Ruby.
Another alternative for a single board Arduino/XBee is John Luciani's ZB1.
$24.95 from Sparkfun Electronics
Buy (Currently out of stock but is available on back order.)