I have never heard this name pronounced with a long i sound although I see several websites and dictionaries that have that as the pronunciation.
Throughout the 1977 film
Iphigenia, shot on location in
Aulis, the name is pronounced as:
if-fəh-ən-yah
http://www.youtube.com/watch?v=Lrv-IuUy5xE&feature=related
Listen to the above youtube clip at 8 seconds for Greek pronunciation.
http://www.youtube.com/watch?v=Lrv-IuUy5xE&feature=relatedAlso in the above clip at 5:26 seconds
This movie is awesome by the way. You will want to use that name even more after you watch it.