I just jailbroke my ATV2 and installed aTVFlash. Perfectly smooth process. Kudos to the devs.
XBMC was another matter. Specifically: setting up an SMB share on an network attached drive (a USB drive attached to a Belkin N+ router). Using "Video >> Add Source >> Browse >> Windows newwork (SMB)" the router refused the connection at first. But a couple of reboots of the router, my Airport Express, and the ATV2, I was finally able to connect to the router and browse for the attached drive. The default login is "guest" (and there seems to be no way of changing this) without a password.
So far so good. I am watching videos in 720HD without any problems at all.
I hope this helps.