I'd like people to be able to add items to a list, but for there to be 1 column in that list only visible/editable by the admin. Is this possible?
If you're using a custom List Definition, you could do the follow (Warning, a lot of work with very little reward)
This will stop the field showing up in Display/New/Edit forms for non-administrators, It will also stop you from adding the field to a view through the UI. I'm not sure about targeting a View to a group though.
As I said though, lots of work, depending on your requirements, probably not worth the reward.
There are a few ways to do this.
Here's a creative one that may work the best/easiest for you.
Don't forget to disallow the normal users from creating their own private views on your list, otherwise they can add the hidden column and edit-in-datasheet and it'll defeat the purpose.
Other methods are a bit tedious, require custom code, and in many cases, if the users are smart enough and determined enough, they can find a way around it. Here's one:
Don't forget about disallowing the view creations as above.
That method doesn't work if your users need to be able to go back and edit the items - unless you use workflow to do that.
In some cases I've gone and created a secondary custom EditForm - just for the admins. It was the only form that actually had the field on it, while the settings on the field in the content type on the list was set to hidden.
Good luck! Tinker a bit and figure out what works best for you.
You can use the following approach: