One of my customers has just asked me this, and I'm not that much into graphic design.

If you were having some posters printed at A0, and you wanted the quality to be good enough (ie close to photo) what res would you choose?

1200dpi?

if you don't have to scale the photo (1:1) 300dpi is good enough. if you have text/logos as a picture (means rasterized/interpolated in photoshop) combined, choose 1200dpi. otherwise you can see the pixels at the edges which is crap.