I've got an Invisible Shield to protect the screen. It's a bit of a faff to get it on, but once on, it's very good. Highly recommended.
Case wise, I have the Griffin Elan 3G case, which is a hard case, but covered in leather. It also has a unique design in that it splits in two, allowing you to remove the bottom half, and put the phone in a dock, without having to remove the entire case (most cases aren't dock friendly).
That being said, I've had the case for about 6 months, and it's beginning to fall apart with the leather coming unstuck in parts. Fixed with a bit of glue - but not what you'd want to be doing really!
If you do buy both of the above, don't put the Invisible shield on the back of the iphone, as it won't then fit in a case.