Development Hardware Options

ATmega128RFA1 Wireless MCU Development Boards

Spark Fun

ATmega128RFA1 Development Board

Dresden Elektronik

Radio Controller and Sensor Terminal boards

Other Platforms

Texas Instruments MSP430 and CC2000-series RF transceivers

An interesting device that is using TI MSP430 16-bit MCU with discrete transceiver chip is Zolertia Z1 board pictured below.

As you can see it feature on-board serial port USB connector and an accelerometer chip. It can be combined with the nice enclosure.

The above box features JTAG, battery holder and a better antenna, but it is not yet available.

Epic Motes

A very interesting board was by Prabal Dutta at UC Berkeley, it has a Digi Connect ME device that can can run a Linux OS as a gateway in RJ45 ("magjack") form-factor.
Schematics for Dutta's board as well as other device from Berkeley labs can be found at Epic Motes Project Homepage. Most these devices are not available for purchase,
however anyone can make their own PCB. Irene Mote would be an interesting
device to use, however it is only manufactured on-demand and sold only in kits.
It would be interesting to see when Epic Mote project switches to TI's single chit "CC430-series":ti.com/cc430/.

Drop-in Interface

Since most of the IEEE 802.15.4 transceivers are interfaced using SPI bus, it would be quite simple to integrate these into an SD-card slot on any embedded platform.
Since SDIO is compatible with SPI and all SD-cards can be accessed using either SDIO or SPI drivers.

To write an SPI driver for a transceiver chip would be quite straight-forward. However there is one concern regarding antenna position, in case if the SD-card slot is on the edge of the board there won't be any interference issues. The design can be quite small (since only one chip is used and power is supplied of the main board). There a few reference to currently announced products avaliable, however none of the products are on the market right now. Thought, there are a few IEEE 802.11 (WiFi) modules in SD-card form factor currently available (for example 1, 2).

Below is the picture of Spectec SDW-820 available from RF Solutions for a relatively low price.

UPDATE:

Spectec also offers micro-SD form-factor zigbee products featuring various TI CC2000-series transceivers.

There is Linux driver already available for AT86RF23x series attached to SPI bus,
so this could be used on a any Linux device which has SPI bus or SD-card slot.
The driver code might require some alteration
if, for example, the application would require occasional use of flash memory card.
Nevertheless, the current kernel version implements an option to configure SDIO UART class.

UPDATE:

Qi-Hardware are working on open-source microSD WPAN card. The source code and design files are available in a git repostitory (according to the changelog, it appears the project is very recent and is currently active).

References

Wikipedia: Serial Peripheral Interface Bus
Wikipedia: Secure Digital Input Output

Linux LoWPAN IEEE 802.15.4 Drivers Branch
Linux LoWPAN IEEE 802.15.4 Implementation Summary