Apple TV works with Ecamm's PhoneView right out of the box. Simply connect your Apple TV to your Mac using the handy USB cable, power it up and launch PhoneView. You'll be able to access mobile user AFC service areas of your Apple TV disk, just as you can with unjailbroken iPhones, iPods, and iPads. That means file transfer capabilities for basic iTunes items like music.
For whatever reason, I have as-yet been unable to see my Apple TV in iTunes. (Update: You can if you accidentally slip into recovery mode. Ooops. I ended up having to restore -- which gives you an error but then the unit connects fine to TVs.) I've tried connecting with power, sans power, etc. No luck. It could be because I have, as yet, not hooked the unit up to a TV. That's because my old HDMI cable doesn't fit the unit and I have to go out and buy a new one. (Update: It's HDMI to DVI, so I need HDMI to HDMI or to buy an audio adapter. Right now, I have no audio.) Until then, the Apple TV won't be able to display on an actual TV. (Update: Now it does, sans sound, I'm afraid.) More to follow on that.
Update: I have not had any problems logging into my Apple account and accessing home sharing.
You can register your Apple TV device in Xcode (it shows up immediately) to the iOS developer portal but you cannot perform any actual development -- yet. Xcode complains that Apple TV does not "support development". Bummer that.
I had no problem pinging my unit after plugging in an Ethernet cable. So that's cool.% ssh root@AppleTV.local
% ping AppleTV.local
PING appletv.local.sadunnet (XX.143.50): 56 data bytes
64 bytes from XX.143.50: icmp_seq=0 ttl=114 time=57.706 ms
64 bytes from XX.143.50: icmp_seq=1 ttl=114 time=56.001 ms
64 bytes from XX.143.50: icmp_seq=2 ttl=114 time=56.854 ms
--- appletv.local.sadunnet ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 56.001/56.854/57.706/0.696 ms
And, of course, I did try connecting -- and failed.
ssh: connect to host AppleTV.local port 22: Connection refused
Bummer that. VNC didn't work either.
TinyUmbrella happily saved my SHSH blobs, but I haven't been able to go further yet. I will post more later on -- stay tuned.