I dunno whether this has been posted, but on the latest developer preview of Leopard (WWDC 2007) you require (and it will NOT install on anything less, much like the Vista Installer):
Read Before You Install.pdf said:System requirements
You must have a Macintosh computer with:
• an Intel processor or a PowerPC G4 (800 MHz or faster) or G5 processor
• a DVD drive
• built-in FireWire
• at least 512 MB of RAM (additional RAM is recommended for development
purposes)
• a built-in display or a display connected to an Apple-supplied video card supported
by your computer
• at least 7 GB of disk space available, or 12 GB of disk space if you install the
developer tools
If you see a message in the Mac OS X Installer stating you do not have enough disk
space to install Mac!OS!X, you can deselect items to save space. To deselect items,
click Custom Install in the Easy Install panel.