"Pro Bono" means, literally, "For the good." Good of what/whom? Society, presumably. Some attorneys do pro bono work, and I applaud them for it.
However, the current investigation of Connecticut's boy-wonder governor has cast light even on the pro bono aspects of practicing law. As someone mentioned, many (most? All?) larger law firms have some sort of pro bono program set up. The attorneys are required to expend 'X' percentage of thier time on pro bono work. That rather defeats the purpose, doesn't it? If you're requird to do it in order to maintain your job, then it's not really volunteer work at all, it's just fulfilling a condition of employment.
Another wrinkle uncovered in the investigation of our governor is that many law firms apparently provide "pro bono" services to select public officials (or their pet causes), but the attorneys are paid for their time by the firm. The court in Connecticut recently rules (correctly, I think) that under these conditions the work is not "pro bono," it's a political contribution.
Ain't nothin' easy these days.