The blog page of the XGPS project has announced that their GPS hardware is ready. The hardware they are using is a Sirf III 20 channels GPS module from USBLOBALSAT and the PIC18F25J10 chip. According to the developers it’s working perfectly and best of all, it hardly uses any power from your iPhone or iPod GPS.
The developers are going to make it public if enough people show interest in the device!

If you would like to make your own, the xGPS team is also giving away the schematics.
I recommend reading the original post on there blog, if you would like to purchase it.