To give you an idea of where the line is, I'm using the T-Mobile Zen theme.
It's a very thin white line that goes from right under the battery symbol on the left, across the top of the time display in the middle, and right below where it says edge on the right.
It doesn't go away after I turn on and off my phone, nor when I switch applications. Sometimes, the line will have a residue of the application that I was just on.

I just recently bought this phone a week or two ago, added a 1GB media card and enabled mass storage mode earlier this week, and added a few phone numbers, but that's it for the most part. I haven't really bothered with my phone yet. Thus, no new applications added.

I called 611 for T-Mobile help, and they told me to reset my phone, which I did. However, I cannot receive any further assistance as of yet because I need to borrow someone else's phone while they help me figure out what's wrong.

Does anyone know what the line is and how to fix it?

Thanks.