10 days is very tight to try to do the itinerary you've outlined, unless you plan to just see the cities from the outside and not see the interior of any points of interest. Then you could technically do 3 days of Rome, 2 days of Florence, overnight train to Munich, 2.5 days of Munich and 2.5 days of Nuremberg, although I don't know if you should, because you'll end up quite frazzled and honeymoon should be enjoyed at a leisurely pace.
If you don't mind leaving Nuremberg out, then you could have 3.5 days of Rome, 2.5 days of Florence, overnight train to Munich, and 4 days for Munich and places nearby for day trips, such as Salzburg (regular train service, journey about 1.5 hours each way) or Fussen (to see King Ludwig's fairytale castle).
Budget to mid-range accommodation would be around €80-€100 per night on average for two, although be aware that in Munich, you may or may not run into Oktoberfest period where accommodation would be very pricey (most likely not though - I've just checked the dates, it starts on 17 Sept 2010 and ends on 3 Oct 2010 - but I don't know if the hoteliers will extend the rate for the month, so just something to bear in mind).
Food (no fancy restaurants but a couple of main meals a day will add up quickly), day transport (although this may be minimal as European cities are very walkable), sightseeing (some entrance fees) - I'd set aside about another €80-€100 per day for two persons. Good thing on sightseeing in Rome and Florence is many sights, especially churches and cathedrals, are free to visit. So that'll help on the budgeting end of things.
Of course, you'll need to account separately for the cost of flights from where you are. The cost of the night train could be offset by the money set aside for a hotel that night and perhaps with a little supplementary charge so it shouldn't change the budget too much.
Hope this is helpful to certain extent.