Hi!
I have the same problem. Once it was the same mistake, in other cases there were errors in package "essentials".

Here is photo of error and installer's log :
http://picasaweb.google.ru/ilushin.alexandr/MacOSInstall
Hardware configuration:
AMD Athlon 64 s939 3500+ (venice - SSE3)
Asus A8V-E SE (VIA K8T890 + VIA VT8237R)
Memory 2x512Mb
Video NVIDIA 7300GT PCI-e
HDD Maxtor IDE 20Gb master (first chanel)
HDD Seagate SATA 250Gb
DVD IDE BENQ DW1640 master (second chanel)
Mouse and keyboard PS/2 (mouse and keyboard are working in the installer)

md5sum is correct, I burned two DVD with some different programs on different speeds (4x and 8x), both of them work identical.
loading work correctly from DVD, after that I make partitions (I try to install both HDD, format and make partitions in Disk Util ,try MBR and GUID), after that I choose, what can I install ( I tried in different ways, including I deleted all che?k boxes), and, when the installing begins, after 16 minutes, the installer writes an error:
"Install Failed
Mac OS X could not be installed on your computer
The installer could not validate the contents of the "Essentials" package. Contact the software manufacturer for assistance."
What can I do in this situation?