Scotsman.com provides a detailed summary of a New Scientist article (original excerpt here) that describes some new research into the history of Roman gladiators:

To amuse the crowds around the arena the gladiators would display broad fighting skills rather than fight for their lives, argues archaeologist Steve Tuck of the University of Miami. "Gladiatorial combat is seen as being related to killing and shedding blood," he says. "But I think that what we are seeing is an entertaining martial art that was spectator-oriented."

I think they're over-simplifying it a bit, because earlier literature (Seutonius, etc.) refers to older gladiator contests as being fights to the death. However, it's quite likely that as the sport evolved and became more and more popular, deadly endings became less and less common, especially among "superstars."

But I think it would be a mistake to come to the conclusion these were simply WWE matches on an outdoor sand-floored ring, or that gladiator death was not possible. The chariot races were probably the only sport more popular than gladiator contests (often the chariot teams formed the nucleus of powerful political parties), and were probably at least as expensive. Yet ancient historians often chronicle how dangerous and bloody that sport was right up until the Christians closed it down in the sixth century AD.

Modern historians have for quite some time known gladiator combats gradually became less deadly over time. The primary innovation this particular archeologist seems to have brought to the table is a set of theories as to how and why this trend ocurred.