I too pick up litter from other people when I see it by the bank, but that's not so often in the places I fish.
Reading the article about cigarette butts linked to by Waggler was interesting. I must confess to trampling the ocassional butt into the mud in the past when fishing, but I'll try to remember to take some kind of ashtray with me after reading that.
Mind you, I was a bit sceptical about some of the 'facts' reported in that article. It was no surprise that tobacco will turn water brown (anyone who's had to clear up the morning after a heavy party will have seen that in empty glasses etc). What I did think was a bit strange was the part which said:-
One study in America in San Diego University showed how a single cigarette butt with a little tobacco left can turn a litre of water a sickly yellow brown colour in one day and kill 50% of the fish swimming in it!
So how many fish were swimming in this one litre of water then? It was a bit of a silly and cruel test to kill those fish because the result could easily be expected. I imagine that if you dropped a rowntree's fruit pastille, a slice of marmite toast, or a milion and one other household articles things into that tiny amount of water with fish swimming in it, then it would kill them.
Sometimes they're over dramatic with their anti-smoking stuff and it spoils their case a bit when it gets ridiculous. Make your case, fine. But don't over-exaggerate.