at first glance it looked fine but i went back and had a closer look at the pic,.... and there is something very odd going on there, are the tiles different sizes or something?? or have you got the hight different on each of the two wall, its hard to tell from the pic.
if the borders are the same height, then do what gaz said first and curve a plastic trim round the inside of the arch. you can hold it in place with panel pins or wee screws if you need to, as long as they dont stick up and interfere with the tile or buckle the trim up. then cut the tiles into soldiers for the inside of the curve.
make sure you you clean it all up really good and seal it all with decent Silicon, make sure it looks really smart and sparkly (go for the wow factor) and it might draw attention away from the missmatched lines.
let us know how you get on, and good luck