Xinerama
Xinerama this, Xinerama that, baawaawaa!!
Too bad. I'm not going to waste my time rubbing your unecological penis enlargement.
However, unlike the crap that is typically to be expected from FOSS, Ion is dynamically modular. So use an internet search engine to find out what others have managed to come up for you and your conspicuous consumer brethren. Remember that any third party modules will void support.
Also note that contrary to popular myth, Xinerama does not equal multihead. X has supported multihead since ages, and Ion does support this plain old X multihead (with separate root windows for each screen), because it is trivial to do so, unlike hacky Xinerama. You could even run a different window manager for each screen under plain old X multihead. All that Xinerama offers and the former doesn't, is windows spanning multiple screens, and moving windows between screens. The former is quite pointless: if your physical screens really merge that well, why do you need to consider them separate at all? That is, what do you need Xinerama support for? As for window migration, it isn't a big issue in applications like TV-out. (It is also only artificially restricted in plain old X multihead, which would've been trivial to extend with that support if people weren't so intent on pushing another ugly hack instead…)