Lastly, a more general question: is now a good time to buy a DSLR, in terms of technological development and price?
I'm probably the worst person to answer this question as I have more than one DSLR. However, I have to say that the latest models are really amazingly good for the money. At the extreme price end, the D3X and Canon EOS 1DS mark III are stunningly good but mega priced, which makes the Sony alpha 900 a real bargain, but what does it do that the alpha 200 doesn't - really it is no better at higher ISOs and being full frame means it is limited to the lenses that expect to see a full frame sensor, so unless you need full frame then you may as well stop at the alpha 200 - 350 end of the scale
On the Nikon front, you can get almost as good a photo in daylight with a D40 as you can with a D300, but if you need the higher ISO low noise capability there is an offering there to upgrade to which will retain investment in the lenses.
I suppose I have always worked on the principle that buying the bottom of the range of anything just means you end up buying another better one later on, so why not save the loss on the cheap one. However, in the case of DSLRs the increasing return as you go up the scale is disproportionate to the expenditure in the same way as buying audio gear. If you can afford a top of the range Naim active system, then why not, but a entry level one will still sound OK.
I think if you only intend to have one camera initially you need to choose the one with the most compatibility possibilities, and retained investment and for me that would be the Sony, as it will use any old Minolta AF lenses and all the new Sony/Zeiss ones. The D40 from Nikon is limited to a subset of Nikon lenses - but if you can get what you want from that range it isn't all that limiting. As a useful alternatibe to the entry level Nikons you could also consider a second hand Fuji S5 Pro or Nikon D200 as these are both capable of using any Nikon glass from AI onwards, and both give better results than the D40 at similar money 2nd hand.
Finally, I think someone else mentioned this earlier, but ergonomics for the end user are important, and if your daughter is like my daughter then she will immediately have a preference for one model over another based on size, feel, useability and weight, not to mention how cool it looks
Incidentally all that will happen in terms of technology and price if you wait a bit longer is that some of the functionality in high end cameras will start to cascade into smaller/cheaper models - eg low noise at high ISOs may get cheaper to acquire.
cheers
Cliff