Secure web site access and PKI Certs

All,

I have access to a secure web site.  It used to require a PKI Cert to
identify the user and then a standard username/password login to
authenticate.

Recently a change was made to the site that allows the supplying of a
PKI Subject CN Fragment to a user "profile" on the site.  In this case,
the certificate not only identifies the user but authenticates as well.

The end result is an "auto-login" feature that in effect, keeps me
logged in all the time.  Anybody sitting at my machine and logged in as
me (Windows XP) can access the web site as me.

At first glance this seems like it's a reasonable way to accomplish a
secure access to the web site.  Installing the certificate as me ties it
to my profile and makes it unavailable to other users on my machine and
since the use of the certificate requires a user to login as me, it
moves the authentication piece from the web site to the Windows domain.

This seems to some extent like "security through obscurity" and also
substituting convenience for security, an all-to-common problem.

Since it's my security-cleared neck on the line, I'd rather be too
concerned rather than not concerned enough.

So I'm asking the collective wisdom of the list to consider.  Is PKI's
single sign-on capability reasonable?  Is this implementation adequate?
Thoughts?  Opinions?  Critiques?

Thanks
Keenan Smith