In photographs all straight lines are curved due to lens distortion. How much they are curved is the significant question. Most of the time it is an insignificant amount that does not detract from the overall image.
Look at a photograph taken with a fish-eye lens you will see what is going on in an exaggerated form, when you take a pic. of something close to the lens front. The perspective is all out which is why it is always best to take portrait shots of people with a longer lens ;) it is much more flattering. All those models in the fashion mags havn't got little button noses, but I bet the photographer has got a telephoto lens!
Or try taking a photograph of water with a perfectly level horizon (I'm ignoring earth curvature) eg the sea or a lake where you cannot see the other shore. Maybe place the horizon a third up the pic as is often done. Now go back to your image editor on the pc and draw a straight line along the horizon.
Some lenses are better than others at controlling the distortion. You can get software to correct the image if the curvature is so much that it detracts from the image but in most cases no one will ever notice unless it is really pronounced.

Message edited on: 11/29/2005 09:38

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html