In July 2000, Apple introduced the PowerMac G4 Cube amidst rumors and speculation on the new radical new form factor.
Due to poor sales, Apple discontinued the G4 Cube only one year later, though even today it remains a popular item on the used market. Uncharacteristically, Apple's press release announcing the Cube's demise noted that "there is a small chance [Apple] will reintroduce an upgraded model of the unique computer in the future, but that there are no plans to do so at this time."
A new unconfirmed report sent to MacRumors claims that the Cube form factor may make a return, though no other details are presently available.
At least one uncertain rumor from May also claimed the return of the Cube... in the form of a limited edition model celebrating the upcoming 20th anniversary of the Macintosh which was introduced in 1984. Readers may remember that Apple's previously released 20th Anniversary Macintosh was released in 1997 to celebrate the 20th anniversary of Apple, itself.