I just got back from two weeks in Italy on Sunday. Wonderful place. Spent some time in Tuscany, the Amalfi Coast (Positano), and a few days in Rome.
Like the previous poster, I was hyper aware (especially in Rome) of my surroundings at all times, and my camera bag never came off my shoulder. Wallet in front pocket. Didn't have any similar dicey situations despite walking (and it is all walkable) everywhere, although I expected it. So I guess I expected the worse, and my fears were not realized, although maybe related.
Italy is full of things you will want to photograph. Everywhere. Almost every church is pitch dark and would be a top 5 art museum/gallery in the US.
I couldn't even think of leaving the D90 home even for the very capable S95. It would have broken my heart at almost every turn. I guess it depends on what you are doing, and where you are going, and what you like to photograph.
Don't get me wrong the S95 is a very capable camera, and outside in good light it will get everything I got, except blowing out some blue skies to white in higher dynamic range situations but....
In Rome alone, let's go through the day at the Vatican. Shooting inside quite a lot, low light. At best, getting 1/15th ISO 800, at F4 lets say. Even the S95 getting into danger territory there (if not already squarely there).
The D90, on the other hand, eats that for breakfast. Take it up to ISO 1600 no problem.
I was shooting mostly with a super wide 10-24 (which didn't require a ton of shutter speed hand held), and with a fast prime (50 1.4).
Saint Peter's Basilica is just breath taking, but again, dark, dark, dark, even at mid day when we were there. Maybe getting 1/30th @ f2 and ISO 800. Breathtaking pictures to be had, if you can get them. Truly one of the more awe inspiring places I have visited.
Same situation in some very cool places in Pompeii or Herculenium. Lots of preserved rooms in really low light that are just amazing. Tons of shots outside as well, where the S95 will excel, but too much to miss.
If you want to come out of those situations with anything usable, D90 all the way. Otherwise, buy a postcard.
Just my thoughts. If photography is your thing, and I imagine it is if you are reading and posting in these forums, then the D90 goes with to one of the most photogenic places on earth.
If on the other hand you can live with yourself, the S95 won't kill you, but I am going to guess a few choice words will enter the mind in critical spots.
As a side tip, setup a private tour of the Vatican, and use one the the services to get in as a group to bypass the long lines for individuals, and get access to some private areas. Saved well over an hour of waiting in line, easily.