Sorry for the late reply. I talked directly to the Firearms Unit in Maine and even got a call back from them. They state to legally carry in Arcadia National Park you have to have a Maine resident or non-resident permit. They state that on their website. That is the way Maine is going to enforce the law.
http://www.maine.gov/dps/msp/licenses/weapons_permits.html
Maine does not honor all other states permit/licenses. Maine law on who they honor states they will only honor states that honor them. That is also on their website.
http://www.maine.gov/dps/msp/licenses/weapons_permits.html
The Maine Firearms Unit is not going to post a list of states they honor. I supplied them a list with references and contacts to check with states that honor Maine. They thanked me but posted nothing. PA no longer honors a Maine Permit/License.
Maine's new law on how they honor other states.
S.P. 313 - L.D. 868
Sec. 1. 25 MRSA §2001-A, sub-§2, ¶F, as repealed and replaced by PL 2011, c.
691, Pt. A, §24, is repealed and the following enacted in its place:
F. A handgun carried by a person to whom a valid permit to carry a concealed
handgun has been issued by that person's state of residence if that person's state of
residence honors a permit to carry a concealed handgun issued under this chapter;