There's no shortage of Warhammer 40K games on the App Store, but not many of them deviate from the strategy genre. Pixel Toys seems to want to address this with Warhammer 40,000: Freeblade, where you play as an Imperial Knight (i.e. a giant chainsaw-armed mech). Works for me!
The newly-released gameplay video is full of all sorts of explosions and action, and is described as playing a bit like Panzer Dragoon. The on-rails implication is a tad disappointing, but then again Panzer Dragoon was an awesome game so it could very well turn out fine. It certainly looks cool, anyway.
Warhammer 40,000: Freeblade will be available for iOS sometime this fall.