When we were in Rome in Oct 2007 we found the VERY best Gelato at Pellachia, Via Cola di Rienzo. The following Oct (2008) they weren't there. Is it possible they have moved to another location? AAA guidebook still lists them (outdated). They are much better than San Crispino and they were VERY rude there and very expensive. We had better Gelato at a little place a bit north of Piazza Navonna, but don't remember the name. Anyone??
There are two gelaterias to the NE of Navona. One is Della Palma, and the other (much better, imho) one is Giolitti. http://www.giolitti.it/english/home.html They're both very close to each other. But the best gelato we found in Italy was Grom. They're a chain of artisan gelato, and they're going to be opening a store right in Piazza Navona, apparently. Yummmmmm.
I agree with the recommendation for GROM. It's the best. My husband gained quite a reputation on our recent RS tour for his gelato consumption, and we would both vote for GROM over anything else.
GROM has opened up in Rome about 125 feet from Giolitti's, close to the Pantheon. It actually sits on the corner BETWEEN Giolitt's and Della Palma. Here's a few other spots: Gelato in Rome. Maybe you're thinking of Gelateria Del Teatro at Via di San Simone, 70?
Teatro gets my vote for the best in Rome!
And FWIW, there definitely is really bad gelato in Italy, so it pays to do a little investigation. We learned that the hard way the last time we were there. I had some chocolate gelato in Rome that was so bad I had to throw it away. It tasted like a petroleum byproduct. It was at Via del Governo Vecchio, #54, at Via di Parione, west of Navona. Blecch.
We didn't try a lot of different gelateria because once we discovered Teatro, we never went anywhere else. It was great!