I have a card named Dissipation Field that reads, "Whenever a permanent deals damage to you, return it to its owner's hand."
If my opponent attacks me with a creature with infect (i.e. Cystbearer) while I control a Dissipation Field, does his creature return to his hand after combat damage is dealt?
Yes. The damage is still damage, and any triggered abilities based on damage will trigger. The only thing different is the effects of the damage, since you gain poison counters instead of losing life.
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.
