Mentions of new iMac models are buried inside the OS X Mavericks 10.9.4 beta that was seeded earlier today.



From MacRumors:

There are three new resource files for power management that mention three new iMacs with model numbers beginning with 15, according to some digging done by Pike's Universum (via TonyMacx86). The current iMac line is made up of models 14,1 and 14,2. The most interesting part is the addition of three new resources (plist) files for power management: Mac-81E3E92DD6088272.plist / iMac15,1 (IGPU only) Mac-42FD25EABCABB274.plist / iMac15,n (IGPU/GFX0/Apple display with id 0xAE03) Mac-FA842E06C61E91C5.plist / iMac15,n (IGPU/GFX0/Apple display with id 0xAE03)

  References to New iMac Models Appear in OS X Mavericks 10.9.4 Beta