If you have, in the past, swapped your system hard drive on your Intel Mac, you might want to check its partition scheme before upgrading to Leopard.
I swapped my MacBook's internal drive shortly after buying the laptop. I formatted it as an external drive, used Carbon Copy Cloner to clone my internal drive, then did the swap. It has been running fine until now. Then, when I tried to upgrade to Leopard, the installer wouldn't let me, telling me I need to repartition my drive.
Apparently, I had used the Apple Partition Map when I formatted the drive, which is supposedly only good for using as a bootable system drive for PowerPC Macs. Leopard, however, will only install (on an Intel Mac) on a drive formatted using the GUID Partition setting, which is designed to be used for boot discs for Intel based Macs. I don't know why my MacBook was working fine till now, but that's how it was.
So basically, I had to use CCC to clone my disk again, reformat the drive, and restore it with the clone, and finally after that, I got to upgrade to Leopard. You can check the partition scheme by going into Disk Utilities, clicking on the hard drive (NOT the partition under it) and clicking Get Info.

