When creating a new lookup column type in SharePoint2010 you are given some additional column options that allow you to pull in multiple field values from the list that is being used as the lookup. Here is a screenshot of the configuration using a lookup to a Document Library-
In addition to the ability to show more of the lookup columns, we are given the ability to control how the list handles the relationship. This means that we can force the target list to not enable deletes for any items that are referenced in this list.
Here is the error screen I get when I try to delete the item in Tasks that I am referencing as a lookup in my custom list:
When you go to enter a new list item you select the lookup value and then the additional columns you selected are populated and available for display in views and on the view properties page for the list.
You can also, from this list select to edit the lookup value. For instance, in the example above, if I select Task 1, the display form for Task 1 is opened in a new window and I can select to modify the values. I click save and then my lookup values are immediately updated.
Now…the real question…how many times did you wish you could do something like this with SharePoint 2007? J
This opens up a whole new realm of possibilities for quickly creating powerful out of the box SharePoint applications!