It is a simple yet important question.
Does a transformed shapeshifter (with the supernatural ability) is resisted by Parma Magica and Might ?
Thanks.
EDIT: Eric posted the rules interpretation of HoH: MC. I do not have the book yet, so I defer to him for showing a better knowledge of the field 
Cheers,
Xavi
No according to HoH Mystery cults
IIRC if you use a spell or item your new shape is resited and you can still be targeted by corpus/mentem effects.
If you are a natural shapeshifter the shape is no being sustained by a spell so it isn't resisted but your fundimental nature is still human so you can be targeted with corpus and mentem.
If you have a heartbeast your fundimental nature is animal so you can not be targeted with corpus or mentem.
If the shape isn't being sustained, does that also mean that it doesn't count for Warping?
That would make the ability pretty nice, though unless I'm mistaken, using the ability always involves a stress die and possible botches. (As does, strangely, Second Sight, which can send a magus into Twilight just for looking at something.)
Anyway,
Ken
Ok thanks for the answers, I was feeling a little guilty to allow this this virtue without magic resistance. (A bear is really scary in combat, so does a shapeshifter against a mage)
It's clearer now 
I'm wondering something... in what category would you class a character with the minor supernatural virtue SkinChanger ? A Natural shapeshifter or a "Magical" one ?
The power come from himself but he need his item to transform.
This is a bit like Verditius needing casting tools , imo.
Unless the item provides a "continuous mystical effect" ,
then it is the same as Shapechanger or Heartbeast.
The effect is active only at the time of transformation.
(No Warping Points for prolonged use)
The "magical cloak , animal skin or similar item made from an animal." (page 48 )
would be affected by magic resistance.
You could rule that when the character is transformed , he is covered by the magical cloak ,
and would be subject to Parma or other might-based resistance if attacking someone with these defenses.
Mystery Cults says it's magical, much like a spell.
IIRC that's correct - which is a bit irritating as I thought there are plenty of folkloric references to shapechangers that don't need an item but can only transform into one animal and never gain the ability to change into other forms.
The current Shapeshifter Virtue doesn't quite mimic this as it allows extra shapes and spending a whole Major Virtue to just change into one animal without an item by choice or to suit a character concept seems a bit steep.
Perhaps an additional Minor Virtue that allows characters with Skinchanger to transform without an item (and be regarded as a natural shapeshifter and therefore not be susceptible to Parma/MR) would be useful? Mystery Virtue or otherwise I could see this working.
Another thought about the item for Skinchanger: can it be a tattoo of the animal, particularly if etched with animal blood and/or bones etc. A bit of a minmax attempt I know - although a tattoo can still be erased/destroyed/removed albeit harder than a cloak or necklace etc.
Just a thought,
Jarkman