As far as I understand a wild shaped druid can revert to its true form with using up a wild shape charge for the day (please correct if this is wrong already).
Does the druid also regain HP when undertaking this action?
The use of wild shape is used when you change into an animal (or plant, elemental, etc.). At this moment you will restore HP as if you slept a night, but no other effect from sleeping like restoring ability damage or regaining spells.
You turn back to your normal form after the maximum time, or if you choose to. You do not regain HP upon "leaving" wild shape.
PHB wild shape description:
... This ability functions like the alternate form special ability, See Errata. ...
PHB errata wild shape:
Wild Shape Player’s Handbook, page 37 Replace sentence 3 of this class feature with the following text: This ability functions like the alternate form special ability (see the Monster Manual), except as noted here. Add the following sentence to the end of paragraph 1: Each time you use wild shape, you regain lost hit points as if you had rested for a night. ...
MM1 alternate form description:
Alternate Form (Su): A quasit can assume other forms at will as a standard action. This ability functions as a polymorph spell cast on itself (caster level 12th), except that a quasit does not regain hit points for changing form ...
PHB polymorph description:
... Upon changing, the subject regains lost hit points as if it had rested for a night (though this healing does not restore temporary ability damage and provide other benefits of resting; and changing back does not heal the subject further) ...
The SRD section on wildshape states (emphasis mine):
Wild Shape (Su)
At 5th level, a druid gains the ability to turn herself into any Small or Medium animal and back again once per day. Her options for new forms include all creatures with the animal type. This ability functions like the alternate form special ability, except as noted here. The effect lasts for 1 hour per druid level, or until she changes back. Changing form (to animal or back) is a standard action and doesn’t provoke an attack of opportunity. Each time you use wild shape, you regain lost hit points as if you had rested for a night.
The first line states that at level 5, a druid can turn into an animal and back again. Considering that at level 5 a druid has wildshape 1/day, the reversion is not considered a wildshape use on itself, but rather is part of the original wildshape.
Furthermore, the final sentence clearly states that when using a wildshape, the druid gains HP (1 HP/HD). Therefore, these HP are only gained when assuming an animal form and not when reverting (as that is not a wildshape use).