So here’s my plan. I have a bunch of little autonomous arduinos doing their own thing. I’d like them all to report to a central location that logs it. It looks like XBee is the easiest way to accomplish this. What I would like to have happen is all the autonomous units connect to a hub that stores the data on an sd card. I just don’t know what I need to buy to do this. I can’t just plug the xbee into the arduino units, but I don’t know what I need to do so. Please help 
You should be able to connect an XBee’s Serial data lines directly to an arduino UART lines.
Xbees would work well for what you want to do. To start do read though all the threads in this forum (Wireless/RF) on XBees. Others are doing what this and there is much info in those threads as well as links to more info.
FYI: there are several protocols that the XBees can run. Study the differences and ask questions before deciding which to use.
waltr:
You should be able to connect an XBee’s Serial data lines directly to an arduino UART lines.Xbees would work well for what you want to do. To start do read though all the threads in this forum (Wireless/RF) on XBees. Others are doing what this and there is much info in those threads as well as links to more info.
FYI: there are several protocols that the XBees can run. Study the differences and ask questions before deciding which to use.
Assuming I bought a couple of [These, and wanted to plug them into arduino’s. Would I need one of those Explorer units for each?](http://www.sparkfun.com/commerce/product_info.php?products_id=8691)
The breakout boards make wiring to the XBees easier as the XBees have 2mm spaced pins and provide 0.1inch spaced holes for either a header or wires. The Regulated Explorer boards have a 3.3V regulator which is handy if your processor board only has 5Volts and it has a diode on the XBee DIN pin to prevent over-voltaging the DIN pin if your processor UART is outputting 5V although this has been a problem for some processors.
Another option is to connect one XBee to your PC or MAC as the central data collector. This would require an RS232 or USB converter. Sparkfun’s USB Explorer does this. You will also need this to configure the XBee parameters using Digi’s X-CTU utility (although it could be done from an arduino interface).
The XBees your linked to are Series 2 hardware modules and run a network protocol the requires one node to be the network coordinator. This would commonly be the ‘central’ node.
The XBee Series 1 modules running straight 801.15.4 is another choice if you only need to do point to point communication.
Do study the pros and cons before buying and read the thread here as these issue have been discussed.