Is there a difference between the protoevangelion and the protoevangelium?

Well both have to do with the Book of James and Genesis 3:15.

Protoevangelion, as I understand it, relates specifically to "the Book of James."
It relates heavily to previously "missing" parts of the Bible and biblical scripture, specific to the "Book of James" in all of its scriptures and gospels.
Basically, it is a fancy way of saying "The Book of James."

It is used in a few different contexts, but the best links are:
http://orthodoxwiki.org/Protoevangelion_of_James
http://www.ecmarsh.com/crl/lost_books/book_of_james.htm

The protoevangelium also relates to Genesis 3:15, but it relates specifically to the First Gospel. The word itself leads back to "the First Gospel."

http://www.tecmalta.org/tft207.htm
http://wiki.answers.com/Q/What_is_protoevangelium

According to these two sources...they both mean "first gospel."

Protoevangelion -- So, God extended forgiveness to mankind, in order to save both the people he loved and the kingdom he planned to build. Theologians commonly refer to this as the protoevangelion or “first gospel.”

Protoevangelium -- This text has become famous as the Protoevangelium (“first gospel”) and the Catechism of the Catholic Church explains why.

http://www.christendom-awake.org/pages/calkins/alliance.htm

http://reformedperspectives.org/newfiles/ra_mclaughlin/th.ra_mclaughlin.redhist.1.html