FWIW, If I can get a good price, I am going to sell my ATV2 and replace it with a Mac Mini. The reason for this is that the ATV2 can't handle 1080 and I seriously doubt the ATV3 will be jailbroken anytime soon.
My advice is to wait until WWDC is over (June 11 - 15th). Hopefully they announce an app store for ATV. Plex has already stated that if apple allows apps on the ATV that they will develop an app for the app store.
I currently have a JB ATV2 w/ Plex, but I would love an ATV3 w/ plex that I dont need to jail break. Hopefully the plex remote would work too.
Panisonic TH-46PZ80, Yamaha RX-V550, Expressvu 9242, Xbox 360, Harmony 880