I'm posting this (November 5th, 2011), because I've found so little guides with Jailbreak information for the Apple TV.
(Here is the Micro USB cable I used: http://www.monoprice.com/products/product.asp?p_id=5458.)
First, back up your SHSH with TinyUmbrella:
1) Disconnect *everything* from Apple TV.
2) Connect the Apple TV to your computer with a Micro USB cable.
3) Locate your device's ECID number.
Open Device Manager, go to Universal Serial Bus controllers, and double-click Apple "Mobile Device USB Driver". Click the Details tab and view "Device Instance Path". Look for the part that says "ECID:" and copy the string after that (up to the underscore).
Mac OS X:
Open System Profiler, go to USB, and locate "Apple Mobile Device". Copy the string after "ECID:".
4) With TinyUmbrella open, click "Manual ECID", enter the display name you want, paste in your ECID number, make sure "AppleTV" is selected, and click "Create Custom Device".
5) Click "Save SHSH".
Now, for jailbreaking, at this time you *must* be using Mac OS X.
The tool used is PwnageTool.
Apple TV firmware 4.1 can be downloaded here:
http://appldnld.apple.com/AppleTV/061-8940.20100926.Tvtnz/AppleTV2,1_4.1_8M89_Restore.ipsw (~253 Megs)
(I found the link to the firmware here: http://www.felixbruns.de/iPod/firmware/.)
Run PwnageTool and just follow the simple, on-screen instructions.
The steps pretty much go like this:
* Select the Apple original firmware.
* Allow PwnageTool to build a pre-patched firmware (it took several minutes to build the pre-patched firmware on my Core 2 Duo @ 2.16 GHz).
* Put your Apple TV into DFU mode (press Menu + Play when prompted).
* Exit PwnageTool.
* Load up iTunes.
* Alt/Option+Click "Restore".
* Select the custom firmware PwnageTool just built.
Disconnect the Apple TV from your computer, re-connect it back to your TV (power, HDMI, etc), and re-start your setup (preferred language, log into iTunes Home Sharing, Netflix, etc).
SSH into your Apple TV to test the jailbreak. It's also recommended to change the default password ("alpine") just like on the iPhone.