Yes -- check out Fodor's Europe forum for walking guides. That forum is primarily independent travelers and its focus to more on seeing the sites/sights than most cruise forums are.
This has also been discussed on the cruisecritic forums -- both Europe and Princess one that I am aware of.
If you have a good map, you can really do it yourself without a guide. The book "Turn Right at the Fountain" has some excellent walking tours in it.
We were in Rome for 3 days pre-Holy Year and saw everything on our own -- had a grand time & great food, wonderful expresso at sidewalk cafes, etc. -- my favorite fountain: the boys with the little turtles as you walk from the Fori market to the Parthenon.
Actually, I hope you're referring to the Pantheon: the Parthenon is in Athens!
The Pantheon, the Spanish Steps, the Piazza Navona (turtle fountains), and the Trevi Fountain are all within reasonable walking distance of each other. All you need is a good map...We took the subway to SPAGNA and did all the above. It took us a whole day but it was our people watching day with iced coffees in Piazza Navona and a late lunch in a local restaurant that had fresh fish on ice in the window and great wine directly from the barrel...
Many street vendors sail maps/tour guides also. One of the best books I've used for Rome is from the "Eye Witness" series. Great walking tours, many pictures, and concise descriptions. I'd also recommend getting a pass for the metro; they're very cheap and easy to ues.