Definitely not wise to flick off as you might on a car. I've flicked off bunch of times on a car, and once I pulled two birdies on an off-duty police officer, and gave dirty look and swearing to another undercover jerk. Suffice to say both did not take my critique of their driving manner so well. But I never flick off anyone on a bike, that's where self control comes in to keep you checked from road rage. And it's about a year and half since I flicked anyone off in a car, coincidentally after I started riding.
Unless you're willing to go above and beyond, and carry a firearm always, and got nothing to lose including jail time or sentence, I would not piss off idiots like that.
Also consider this, they have a false mentality that they rule the road, and since you retaliated, just by the size of your vehicle, he thinks you're a bug waiting to be squashed, or at least put a good scare in you.
Although I have to say the last bit was good justice, but it could've been worse.