First, to glennzo: Apple switched from PowerPC to Intel CPUs about five years ago, so Macs that deserve to be called "modern" have Intel x86 or x86-64 CPUs in them and can run the same versions of Fedora that most PCs can run.
As to the problem, if the boot process is slow but the computer eventually boots, it could be that there's a partition table problem, but I can't be positive of that. I've heard that damaged hybrid MBRs can produce this sort of symptom. This probably wouldn't account for the problem if the computer isn't booting at all, though. In that case, it sounds more like a boot loader problem. Intel-based Macs use the Extensible Firmware Interface (EFI) rather than a Basic Input/Output System (BIOS), which is what most PCs use; however, Apple's EFI implementation includes a BIOS emulation mode to enable Windows to boot. It could be that the partitions are set up in such a way that BIOS emulation mode isn't being triggered, in which case you'd need suitable EFI boot support, which might not have been set up by the Fedora installer.
In any event, it would be helpful to see the partition table on the computer. I recommend you download and burn PartedMagic
or System Rescue CD,
boot it on the Mac, launch a text-mode terminal, and type the following two commands:
fdisk -lu /dev/sda
gdisk -l /dev/sda
These commands will display the disk's Master Boot Record (MBR) and GUID Partition Table (GPT) partitions. Post the results back here; it'll help reveal how the Mac will try to boot and what options you might have to fix the problem (assuming it's a software issue).