All I have to do is point to the 'M' rating on the video game and Ryan dropping an "ah, **** me" in the test footage and I'm pretty sure if they keep to the character, the MPAA would want to slap an R on it. Even if a few F-bombs are becoming prevalent in PG13 films, between that, 'Pool's other language, and all the violence... Yeah.
Incidentally, I saw a bride and groom Deadpool at Emerald City Comic Con this past weekend. The bride was holding a sign that said "My anaconda don't want none if ya ain't got guns, hon" and the groom's read "I put a ring on it". So Deadpool I was giggling for the next ten minutes or so.
Thing is, an R rating is no real deterrent to younger viewers. It tends to just mean parents go with (after making sure you approve of your kids watching such fare). The rating only means under-18s are restricted, not banned. That whole block -- PG, PG13, and R -- all are just varying degrees of "parents, make sure you check this film out and are okay with your kids seeing it".
--Jonah