I just saw in this video that there is an Charging adapter cable (at 1:35 mins) included in the retail box (as shown in this unboxing video from the Maemo summit).

With this I guess we can reuse the old Nokia wall chargers for the N810 or any N series phones and convert that to a USB charger for the N900. Pretty sweet.

And this means you obviate the issue of connecting to a computer and can use your existing wall chargers.
I do hope this is included in all retail boxes.