Misinformation
After reading all 8 pages so far, there have been a number of comments that I would consider misinformation. I'm not an insider, and I haven't had access to any Leopard betas, but I am a software developer.
Someone argued that Tiger was 64-bit, so you couldn't attribute much or any of the disk space requirements to Leopard's 64-bitness. This is not really true. Tiger was a 32-bit operating system (whether on Intel or PPC), but it included some low-level libraries in 64-bit versions so you could write non-GUI (command-line and daemon) apps in 64-bit. (Apple's marketing really made the most of this!) There are NO 64-bit GUI apps on Tiger, because the GUI libraries (carbon, cocoa) are 32-bit. You might occasionally find a 32-bit GUI app that communicates on the sly with a 64-bit faceless app in the background, but this was rather messy.
The big change with Leopard is that the universal binaries for most of the system frameworks and toolkits are now "more universal": they are compiled for 32-bit and 64-bit, as well as Intel and PPC. (Well, the GUI portions of Carbon are still 32-bit only, but most of everything else is 32-bit and 64-bit universal.) So I do think the more complete 64-bitness of Leopard is probably a significant contributing factor in the enlarged disk space requirements.
The OS is also localized for several additional languages/cultures, so that's part of the disk bloat as well. There will no doubt be ways (either from Apple during installation, or from 3rd parties afterwards) to trim out some of the unnecessary code & resources.
As far as the much-ballyhooed "backdrop" feature not running on PPC. As others have noted, SSE3 (and even SSE2) have a few features that AltiVec does not, and AltiVec has a few features that SSE does not. Apple's Accelerate Framework could only be either a) wrapping only the features that both architectures have in common, or b) providing less-performant alternate methods for operations that one architecture has built-in but the other does not. So just because Apple released this framework (back with Tiger, incidentally), doesn't mean that it would necessarily provide enough functionality to make backdrops work on AltiVec.
Lastly, on the processor speed issue. Clearly Apple's website says an 867 MHz G4 or faster is "required." But do we know for certain that Leopard will refuse to install on something slower? Some posters have stated this is the case, but are they speaking from experience or assumption? I don't know one way or the other, but I do know that many Apple software packages (such as iLife '05 and '06) have stated requirements of 800MHz or 867MHz G4, but have installed just fine (without any hacking or complaint) on both my 450MHz and 500MHz Dual G4s. Of course, certain features run so slowly as to be impractical, but they still run. I wonder if Leopard's stated speed requirements are more of the same: more of a disclaimer than an actual requirement? (i.e. "Slow machines are not supported; don't try any class action lawsuits saying we slowed down your machine, because we told you so, etc.") I'm curious to find out on this one.