Does virtualization imply no ACCESS_DENIED errors ?

Hello,

From Vista documentation, it seems whenever an application tries to write
inside a protected part of the filesystem, virtualization will kick in and
the write would succeed, through redirection. Does that mean all writes
inside "Program Files" etc. would always succeed, and the application in
question would never get the ACCESS_DENIED error it deserves ? Wouldn't
that prevent developers/testers from finding out such problems in the first
place ?

If virtualization is only meant for "legacy applications", how would Vista
decide whether an application is a legacy software (and therefore
virtualization should kick in), or a Vista compatible one (so it should get
the ACCESS_DENIED error) ?

Thanks,
Saurav.