By way of the "sufficiently advanced technology is indistinguishable from magic" approach. The Jedi can force jump, the Trooper can rocket jump. The Sith has force lightning, the Bounty Hunter has a mega-tesla gun. The Jedi has force push, the Smuggler has gravitonic warp grenades.

...you get the idea. Obviously we don't want cloned characters and I'm sure Bioware will be careful about that, but any single ability can be plausibly assigned to any class by changing its name and particle effects. I wouldn't think class imbalance issues would be too difficult to address using this approach.

Then again, who knows. Maybe we WILL have 90% Jedi in the first few days. If that happens, well, all I can say is that I liked my time playing a protection-spec warrior in the early days of WoW. I'll just roll the least popular SW:TOR class, and enjoy being worshiped by desperate group leaders.