Dead space would just be assigned to the nearest possible key anyway so this isn't much gain.
Now, if you had tessellated triangles, that might work. Right now, the keyboard works by row and column. I suspect it's more common for the iPhone to trigger "f" as "d" or "g" rather than "r" or "v": in other words, detecting side to side is harder than up and down. That suggests to me that if you tessellated triangular keys together, how far up and down you are would help determine whether you were going for an upward-pointing key or a downward-pointing key. This isn't very clearly stated but I hope it makes some sense.
Now, if you had tessellated triangles, that might work. Right now, the keyboard works by row and column. I suspect it's more common for the iPhone to trigger "f" as "d" or "g" rather than "r" or "v": in other words, detecting side to side is harder than up and down. That suggests to me that if you tessellated triangular keys together, how far up and down you are would help determine whether you were going for an upward-pointing key or a downward-pointing key. This isn't very clearly stated but I hope it makes some sense.