The Drupal 7 core includes an administration interface theme, Seven, which is enabled by default in the standard installation profile. Whatever the theme used for user-facing pages, administrative pages will be rendered using Seven. See the following in /profiles/standard/standard.install:

  // Enable the admin theme.
  db_update('system')
    ->fields(array('status' => 1))
    ->condition('type', 'theme')
    ->condition('name', 'seven')
    ->execute();
  variable_set('admin_theme', 'seven');
  variable_set('node_admin_theme', '1');

You can of course change this easily, either in code or the administrative settings, but by virtue of Seven being the default out of the box - and a pleasant, clean theme to work with - a majority of sites and developers keep it around.

Over the years in which Drupal 6 ruled the roost, a great many custom administrative themes were developed in order to scratch various itches - to make some tasks easier, to rearrange the UI, to make the administrative interface flow more smoothly for some applications, and so forth. Some are quite intricate and ingenious, especially those intended to be used as base themes and which allow administrative management of much of the child theme's properties.

Thus when you undertake a project to upgrade a site from Drupal 6 to 7, the odds are good that one of these custom administrative themes is being used. The major administrative themes have all been ported to Drupal 7 by now, and the average client will want to avoid changes to the look and feel of the back end - since a change of theme might require time to retrain less sophisticated administrators and update internal documentation - such as screenshots in process descriptions.

There are lurking problems, however. The issue that you tend to run into here is a consequence of the fact that module developers overwhelmingly work in Seven. So if a module has any sort of administrative user interface to speak of, it was most likely tested and worked on in Seven. Therefore for themes that are very different from Seven in approach or layout - such as Cube, to pick one at random - you are going to find all sorts of lurking user interface issues in Drupal 7. These include plain old ugliness; wrapping issues; lack of space for wide content; blocking placement of links or modal dialogs; and a range of others. Views is a good example of a module that exhibits issues in contributed administrative themes in Drupal 7, but there are others that are far worse, and even core pages such as block administration or content lists become problematic in narrow themes like Cube. After all, core development and testing largely took place using Seven as well.

It'll take years for this sort of thing to work its slow way through the bug pipeline, even for major modules, as it isn't a big priority for the development community. Drupal 6 has had those years, and lacked a default administrative theme in any case, so the situation there is far more manageable - and hence the potential surprise when upgrading a site to Drupal 7.

The same scenario of user interface issues also exists when adding new functionality to a Drupal 7 site with a contributed administrative theme - the odds are good that usability problems will crop up if new administrative UI has any significant complexity.

The bottom line is to use Seven as the administrative theme in Drupal 7 if you can; that will probably continue to be good advice for some years. If you can't convince your client that this is a good idea, and they still wish to forge ahead with a contributed administrative theme, then you will have to budget for additional cost and time to deal with the inevitable issues. That is balanced against the cost of dealing with a change in administrative theme, of course, but it might be as much as several man-weeks for a major project with fifty or more modules involved.

Either way, in the early stages of a project it is an excellent idea to throw the proposed administrative theme and all major modules onto a test server and walk through the administrative interface to see what it looks like. If you don't do this, you're only setting yourself up for a series of unpleasant surprises.