Yes and no... I was trying to clean up the several places in code where we'd hardcoded that root = 1.  I found there to be three types of instances:
- get the root item model (Item::root())
- test an item model to see if it's the root (now done with Model_Item::is_root(), as in $item->is_root())
- test an id to see if it's the root (commonly used to check an item's parent, now done with Item::ROOT_ID)

While it's likely a small performance gain (don't need to load an ORM model), my real reason for doing this was that it seemed to be the cleanest way to do it - define the constant once, then use it in Item::root() and Model_Item::is_root().

Thoughts?