I was speaking about natively installing ext2/3 FS and modifying windows to use it would be a *HUGE* tweak including the way windows searches for files and displays them. Windows I don't think is built that flexible to allow you to just plug in which FS the kernel uses and abstracts.
The kernel sees an abstract view of the filesystem through the Installable File System. That's what allows drivers to operate, such as the ext2/ext3 drivers.
The real trick to it though would be to trick the system into believing that the Ext2/ext3 systems are built-in components. Maybe someone should reverse-engineer the WinFS setup while it's still in beta and see what changes it makes and how it's registered. Furthermore, depending on how the driver system is constructed, Explorer might not be able to identify that it's an ext2 drive in Windows Explorer (it might alternatively say "Unknown partition" or something similar). That depends on whether the driver or the explorer.exe program provides the partition type name string. I'm almost positive it would be impossible to format a drive to ext2/3 without modifying explorer.exe or providing a new shell (that functionality is built into explorer.exe itself).
I don't think it's THAT different though. The file system still shares similar structures with Windows, such as folders. It's just that / would become a root folder for a disk drive. ntldr would need to be modified to accomodate that.