Did you use the whitepaper or the extranet tool to set your portal to be
extranet?

I didnt use any extranet tool (or I am aware of any). As per whitepaper
is concerned.... most of the information is irrelevant.

The exact problem is that the site  is working perfectly via intranet.
Its also working with the IP Address I assigned. The only problem occur
is when I access it via external  fully qualified domain name... i.e.
http://subdomain.domain.com/workspace

The error  number is:
-2147217895

I have searched all microsoft support site.. but no fun

You can either use the Extranet tool (look at link below) or use the
Whitepaper. I've had more luck with the Whitepaper. First, did you setup
a web site  (not a virtual directory) for your workspace that can be
accessible from outside? If you didn't, then it won't work. It will only
work for intranet. Give either the tool a try or the Whitepaper and let
me know how it turns out.

www.microsoft.com/.../spsextranet.asp

The utility  is just great. I will let you know whether it worked or not.