The Arena have said it was outside.
I know (and reports immediately afterwards are often confused and contradictory). Some eyewitnesses have quite specifically mentioned one inside, like the guy early on who was stood in the doorway and said he was blown backwards from the explosion.
edit: a local BBC reporter on BBC News now, quoting an eye witness inside the foyer talking about being hit and there being 'nuts and bolts' all over the place, possibly corroboration of the increasing rumours of a 'nail bomb'.
Who knows. Horrible, whatever the details.