I think it's really very difficult to take the personal is political line on this one. As much as I take Belle's point, we are living in a world that values beauty. Yes, that is wrong and, yes we need to emphasise our children's more important qualities, but to avoid commenting on a child's beauty is confusing in a world that values that trait so highly.
As others have said above, when parents do not tell their children they are beautiful, they grow up thinking they are not, which is damaging to their self-esteem when wider society deems that so important.
We are not in a situation yet where comments on visual appearance are simply statements of fact. For many (girls in particular) those comments are statements of worth. This is, of course, wrong, but while it is a fact of life, I would be uncomfortable trying to change the world at the expense of my children's self-esteem.
I would prefer to tell my children that they are beautiful, while emphasising that there are other things about them that are more important: 'Yes, you are beautiful, but I am more proud of your brains / strength / kindness'
Also, as I have said above, I do see a difference between beauty and what is 'pretty' and I think we need to reclaim the word. We can only do that by using the beautiful label for less conventional qualities. My DD's pale skin and freckles are beautiful and I would like her to accept that. If we stop commenting completely we risk children getting their messages on beauty from elsewhere and reaching the conclusion that their less-than-conventional qualities make them less beautiful.