Question : Can Ruby pull the current logged in user id from an xp system?

We are developing a Ruby on Rails application. The app will be used with IE7 on a domain by users with active directory accounts. We would like for the app to recognize the username of the person currently logged into their xp machine. Knowing that Ruby is executed on the server, I wasn't sure if this was possible. We want to avoid a separate login page if possible. Once the user logs in to their system, we don't want them to login again for the web app.

Answer : Can Ruby pull the current logged in user id from an xp system?

This isn't really a ruby or rails question.  You need some javascript that can read the local user or maybe an ActiveX control.  Like you said, the app is on the server, so you need something running in the web browser.  

Once you had it, your entry page to the application would just redirect to the login "submit" URL with the appropriate username.

Now, I have to run out the door so I can find you the appropriate javascript, but I'll check it out later if no-one else has found it.

Note that you'll have no login security on your web app unless you include more information (include the computer name?)  You probably can't get their password to submit it too...