I too think it will be best to leave Arnie out of it.
As has been shown with the past two films, there are other types of Terminator machine out there.
Perhaps a cameo like he did in T3, as the soldier who obviously inspires the design of the Terminator cyborg.
You cant have a terminator film without arnie, it would be like the a team without B A. or knight rider without kit. it's just not right. but i wouldn't believe imdb to much when it comes to up coming movies, they tend to get it wrong. gregs preview on yahoo movies is better.