Only the system.img and userdata.img files within the HTC ROM files (PB00IMG.ZIP) are formatted in yaffs2 - the rest are just raw (binary) data blobs.
I'm not sure that's what the OP meant by "ROM", but whatever.
Note that the Nandroid backup files "system.img" and "data.img" are also YAFFS2 formatted - so if you have been using Nandroid, you can extract saved Nandroid backups (for instance if you are longing for an older version of an app that you formerly used, but hate the version available on the Market).
A couple of notes about using "unyaffs" - it does not preserve time-stamps, file modes, or ownership information, even when running as root on a Linux box (the sources can be fixed up, though - *cough, cough* ). Certainly ownership and file modes/permissions will not be preserved when using a Windows version of the tool.