Thanks all - that's really interesting. The reasoning behind the possible Dutch bike was to make enough space between rider and handlebars for a child in a front seat. I'll have to check whether we've got enough space on our current bikes to do that. The idea was that we'd need a rear seat anyway at some point when he's bigger, so we might as well get one now and swap it as and when we get a new bike.
It's an interesting point about a harsher ride if it's rack mounted - I hadn't thought of that. It's certainly something worth considering. Fortunately, we've got pretty smooth roads around here, and cloth nappies provide their own special extra suspension I wonder if it fits in the category of the 1001 things where you can potentially harm your baby, but in reality the dangers are small. To those who have frame- mounted rear seats - have you found them stable enough, or are they very wobbly? It strikes me that hanging 15kg+ off two long arms would have a lot of flex.
To those concerned about backpacks - have no fear! I hate wearing a backpack on a bike anyway, and I'm sure that of one of us has the baby, the other will be carrying the luggage.
Regarding the trailer - it seems to be the most popular option locally, but I find they take up a lot of space on cycle paths, and I don't like the extra length. Add to that they're pretty expensive, for just a little bit more money we could get my wife a decent new bike, which will hopefully still be used long after our little guy is riding himself around!