How can I download a distribution? Am I right in thinking that the Drupal distributions that I can download from http://drupal.org/project/distributions are a standard Drupal core download with a load of other stuff added at /profiles?

Anyone know if there are any plans to create a dev branch of distributions that would automatically update the core version that the distribution is running on? Currently it seems that the distribution runs on whatever core version its developer has chosen.

And how can I use distributions with Drush? Is there an equivalent to drush dl drupal that will download a specific distribution?

And, finally, is there any difference between an install profile and a distribution?
A distribution is a collection of Drupal modules configured to perform a certain function.  An installation profile is a script that will configure Drupal at install time.  A distribution always has an installation profile in it, but not every installation profile will be a distribution.  An installation profile can be distributed without the modules it uses; in that instance, you can use drush make to download them, as installation profiles include a make file.

Drush dl will download distributions as well as Drupal core and contrib modules (e.g. drush dl commerce_kickstart).  Distributions are updated whenever the developer chooses to update them; of course, you can always run drush pm-update on your copy of your distribution, but there are testing concerns -- occasionally, updates will break things.  This is no different with distributions than your own Drupal install, but in the case of a distribution, presumably the dev will test and fix problems before rolling out an updated release.