I am not sure what you mean by  IIS replacement.
I guess you mean a feature in Whidbey that we can develop ASP.NET
applications without installing IIS, is it? If yes, then this feature
should be a Whidbey only feature, and it can't be used with vsnet2003.

In addition, if you want to similiar fuctionality as whidbey does, you may
try "Free ASP.NET Web Development Tool -- Web Matrix" at:
http://www.asp.net/Default.aspx?tabindex=0&tabid=1
Please note that this tools is a community product. It is free and not
supported by Microsoft, you use it at your own risk.  

Thanks,
Rhett Gong [MSFT]
Microsoft Online Partner Support
Get Secure! - www.microsoft.com/security

This posting is provided "AS IS" with no warranties and confers no rights.