I generally opt for simplicity and just check the URL to get the ID when I need it rather than adding yet another plugin ;)

But there is at least one plugin that does this, Reveal IDs for WP Admin, that I have used when working with other designers who want an easier way to get these IDs.

It seems like they added a lot of flexibility in the backend for the end user with 2.8, it seems like the option of IDs would be a reasonable option to add as well.