I get this question a lot from the Why-Not-WPF-Crowd so I thought I would post it out there.

.NET Framework 4.0

  32-bit 64-bit Mac OS
Processor 1 GHz 1 GHz N/A
RAM 512 MB 512 MB N/A
Disk Space 850 MB 2 GB N/A

Silverlight 3.0 - link

  32-bit 64-bit Mac OS
Processor 450 MHz 450 MHz 1.83 GHz
RAM 128 MB 128 MB 128 MB
Disk Space 4 MB 4 MB 7 MB