the problem is your use of the "user instance=true" and "AttachDBFilename=DotNetNuke.mdf" strings. User instances are different from normal sql in that they only operate against a local installation of sql express (e.g. .\sqlexpress). They also don't need usernames and passwords as they open in the context of the administrator. 

http://www.connectionstrings.com/?carrier=sqlserver2005 is a good reference for sql connection strings (check out the SQLConnection(.net) section).

In your case if you're looking to use a remote SQL server via windows authentication, your connection string will look something like (you obviously have to create a database and grant the user permissions as well)

add key="SiteSqlServer" value="Server=172.23.140.53;Database=yourdatabasename;Trusted_Connection=True; 

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Xnowman:

The site Cathal recommended is very good, make sure you use the string examples for .Net though.

Also, if you are using the database on the local machine do not use the real IP address. I'm not sure but in SQL2005 it may not work because it takes it as an external connection request, which are not allowed by default. Use ".\SQLExpress" or "localhost\SQLExpress". For SQL2000 in the local machine use "(local)" with the parenthesis but no quotes. This bypasses the overhead of going through the whole TCP/IP stack to talk to the server.

Good luck.

Carlos