Veteran Mac developer Peter N Lewis (of Keyboard Maestro and Interarchy fame, though the latter was spun off a few years ago and is under different ownership) has been producing his own photographic desk calendars using Perl scripts and the ImageMagick open-source graphics utility.
Mr Lewis's company Stairways Software has now productised the software as Desktop Calendar Maker, and made it available through the Mac App Store for $A12.99/$US9.99.
As with commercial calendars, the app puts one user-supplied photo onto each month's page. The output is sized for use in CD calendar cases (slightly modified CD jewel cases that allow the lid to fold back beyond the usual 180-degree limitation).
Instructions, including sources for the cases, are available here.
It is not apparent from the description or documentation whether Desktop Calendar Maker integrates with iPhoto. If not, Karelia's iMedia Browser might be a useful accessory.