Well, not so fast. Gerald Smith's response points out that Moroni did not become the capital until 1876. It appears that maps and gazetteers of Joseph's day (which he probably never had access to anyway) often neglected the Comoros Islands, and didn't show Moroni at all. The case for plagiarism doesn't make much sense.
On my mission in Switzerland, I ran into a couple of Italians with Moroni as their last name. (No, I didn't accuse them of plagiarizing from the Book of Mormon.) It was a reminder of how similar words can crop up by chance in various parts of the world. Coincidences happen - and it was fun knocking on Moroni's door.
When two names seem to match, some sort of plausible connection or nexus still needs to be established before one can assume there is any relationship.
Now if you want to explore another interesting match, how about the ancient Mayan city Lamanai in Belize? (Sounds like Lamoni, a Lamanite king in the Book of Mormon.) I read somewhere that is one of the few Mesoamerican cities that has kept its ancient name, but I'm not sure about that. Is this another chance coincidence?
Update, March 2006:On the issue of Lamanai, I have received email with an interesting quote from the Journal of Field Archaeology:
Our choice of the site for intensive investigation was based on the presence there of a 16th-century secular Catholic church, the existence of which was first noted by Castells, who incorrectly identified the structure as Pre-Columbian. Castells' description of the church was, unhappily, the basis for Thompson's mention of a structure with round portal columns at the site, possibly reflecting Central Mexican influence in Western Belize. Though the church had been the source of some confusion, its presence demonstrated that Lamanai was inhabited in early historical times, and the possibility clearly existed that the occupation in that period represented the upper end of a continuum from the Classic (3rd to early 10th centuries A.C.) or earlier. There was ample evidence, in the form of obviously complex, large structural remains, to indicate that the site had probably been an important center during the Classic, so that excavation could be expected to provide insights into developments in the Central Maya Lowlands over a considerable period. Though the locale is generally known in Belize as Indian Church, a name apparently coined in the early 19th century, Lamanai is in fact one of the very few Maya sites for which the ancient name is recorded. It appears on a church list of 1582, and the site was visited and very sketchily described by Fathers Bartolome de Fuensalida and Juan de Orbita in 1618. (David Pendergast, "Lamanai, Belize: Summary of Excavation Results, 1974-1980," Journal of Field Archaeology, Vol. 8 No. 1, [1981) pg. 29-30)David Pendergast was involved in excavations at Lamanai beginning in the 1970s.