Even the full-presence suites don't quite work once you start going over any distance. The lag is just enough that the bit of your brain that processes NVCs gets a little out of sync with your thoughts.
According to cisco design guidelines[1] , 150 msec is acceptable network latency(not including video encoding/decoding). Also [2] talks about 300 msec total latency about the limits for remote surgeries.
Even if we take 150 as our guiding figure, if we assume direct fiber optic connection between sites, the maximum distance would be 45000 kilometers, which can basically connect any 2 points on earth.
So theoretically at least, it's possible. Achieving good latencies in real life is much harder though :)
