I don't see the point: the Lumia 900 is a great smartphone, the best over there, if you also consider the apps Nokia included (Nokia Drive, etc).
WP8 devices are not here yet, we only have a (leaked) emulator. it will take time to have a stable WP8 smartphone with plenty of apps that really take advantage of the underlying OS.
do you want to wait having a black/white old phone or a shiny Lumia with all the best stuff you can actually have? you know the answer.
as soon as you will have a WP8 device in your hands, this site will talk about rumors for WP8.5, the new iPhone OS and Android next generation devices, then you will start looking at your WP8 device as first gen again.