I don't believe this is a bug, I think it's by design. I ran into this a few months ago myself.
Consider an application that creates a temporary, working copy of the file you just opened. As you're working, your changes are written to the temp file. When you're finished and choose to save the file, the application deletes the original and renames or copies the temp file to the original file name. It's not terribly common, but it's not uncommon (many old and simple applications work this way...text editors and the like).
In the above case, every time you saved a file, its creation date would always match last modified!
I dunno what kinda junk is going on behind the scenes, perhaps somebody with some insight will provide more details. As an aside, some file systems don't store creation dates at all, only modified and accessed dates (ext2, for instance).
Update: I found the following from the writers of xxcopy, at http://www.xxcopy.com/xxcopy15.htm , which might apply to what you're doing:
Since the use of the File-Create date
has serious problems, we generally
discourage the use if this date
Problems with the file creation date
The problems of the File-Create date
can be traced back to the
inconsistency in Microsoft's various
file management utilities. It seems
that the purpose of three distinct
variations in the file date values
were never clearly defined by the
designer of the feature. We as
software developers have not come
across any official documents on this