I have a card named Dissipation Field that reads, "Whenever a permanent deals damage to you, return it to its owner's hand."
Mark's answer above is correct, but I wanted to expand on this and couldn't do it properly in comments.
They don't count as damage, but they do count as dealing damage if their source is from a creature with the Infect static ability. The "deals damage in the form of poison counters" is probably the reminder text for the Infect keyword:
Infect (This deals damage to creatures in the form of -1/-1 counters and players in the form of poison counters.)
There is a ruling on Infect:
Damage dealt to a player from a creature with infect does not deal damage as well as poison counters.
Damage is assigned as a number and can be prevented as normal, but when it is dealt to a player it turns into poison counters instead of reducing their life total. Damage has still been dealt, provided it was not reduced to 0.
On the other hand, a creature with Poisonous N deals damage normally and has a triggered ability:
Whenever this creature deals damage to a player, that player gets N poison counters.
In this case damage is dealt as damage as long as it isn't reduced to 0, and that triggers the N poison counters regardless of how much damage was dealt.
In either case, damage is dealt, whether or not it results in loss of life.