Case Status: Case closed. To test Hera's claim, Zeus formed a cloud nymph named Nephele and shaped her to look like Hera. Ixion then came along and had his way with Nephele, proving Hera's claim. Ixion was sentenced to spin eternally on a flaming wheel as punishment.
Addendum: Not long after, Nephele gave birth to the tribe of centaurs on Mt. Pelion. (We're not sure how human + cloud = horse-men, but whatever.)