It used to be considered so - and a horse could be marked down for it. Now, however, it's recognised as being something that could just be peculiar to that particular horse - and not a sign necessarily of resistance or pain.
my mare swishes her tail all the time, and she hasnt got any back problems/pain as shes is checked regualarly. some horses just do it more than others.
My mare does it all the time and ive recently had the back man out to check her over, and all is well.
She seams to do it as i ask her to go up a gear and when jumping so i think its a sign of her enjoying herself.
Could be a sign of concentration, although that is just a guess! I know Priceless, Ginny Elliot's event hoss did that over fences just because he was concentrating so much - his tail shot vertical into the air bless.
I was always led to believe that tail swishing was a sign of agitation and stress. I don't have a horse that does it so I can't comment from personal experience.
Antifaz does this alot... more when he's having to work quite hard! It was a worry at first, but when his back was checked he was found to be totally fine.
We had this conversation the other night watching my instructor schooling my horse. She swished through being cross or getting agitated (not side to side), whereas the dressage horses move their tails from side to side which I read in the BRC mag is also a sign of engagement.