Apparently there are hundreds of these round carvings all over the country, ranging in size from centimeters to meters. They are now used as lawn ornaments by the rich! Also, according to this archealogy thread, as doorstops at a tourist cabana. Like all stone artifacts, they aren't reliably dated, and could have been "created" anywhere from AD 200 to 1500.
"For me, the spherical shape probably evolved in response to the need to move these objects. After all, spheres roll in all directions with minimum resistance. We find spheres weighing several tons atop 100 m high hills, so transport was an important consideration," says John Hoopes in the email thread. This argument seems, to me, a bit circular (not to mention spherical). I mean, primitive man moved monoliths to Salisbury plain, without having them spherical. (On the other hand, maybe the ancient English just weren't smart like the Costa Ricans?)
The photo of the stone ball is credited to Erin Bradner, who may or may not be the same one I was searching for at Autodesk. If she is, I think we'll get along just fine! (She also seems to have a respectable publication record in the field of computer-mediated communication, where my dissertation fit as well.)