I've seen this question come up a few times.  Doug Neumann, our PM, wrote a nice explanation in the Team Foundation forum (http://forums.microsoft.com/msdn/ShowPost.aspx?PostID=70231).

It turns out that this is by design, so let me explain the reasoning behind it.  When you perform a get operation to populate your workspace with a set of files, you are setting yourself up with a consistent snapshot from source control.  Typically, the configuration of source on your system represents a point in time snapshot of files from the repository that are known to work together, and therefore is buildable and testable.

As a developer working in a workspace, you are isolated from the changes being made by other developers.  You control when you want to accept changes from other developers by performing a get operation as appropriate.  Ideally when you do this, you'll update the entire configuration of source, and not just one or two files.  Why?  Because changes in one file typically depend on corresponding changes to other files, and you need to ensure that you've still got a consistent snapshot of source that is buildable and testable.

This is why the checkout operation doesn't perform a get latest on the files being checked out.  Updating that one file being checked out would violate the consistent snapshot philosophy and could result in a configuration of source that isn't buildable and testable.  As an alternative, Team Foundation forces users to perform the get latest operation at some point before they checkin their changes.  That's why if you attempt to checkin your changes, and you don't have the latest copy, you'll be prompted with the resolve conflicts dialog.