Drupal 6.x vs 7.x for Multiple Domains and LDAP
This one is easy: go with 6.2.x!

Despite being new and exciting and all of that, Drupal 7 still is very lacking in the stable contrib department. Also, despite the huge number of modules that adopted the D7X pledge, a lot of important and frequently-used modules still don't have stable versions (ahem, Views! say what you want - "beta" and "dev" are not stable enough for enterprise purposes)

Drupal 7 also introduced some API changes that, despite not being huge, still mean that there's a small new learning curve to developing Drupal 7 modules.

Drupal 6, on the other hand, is more than stable and has plentiful offerings for modules including LDAP integration and anything else you might need. Since it's been around for a couple years now, there's no shortage of experienced developers who can aid in implementing the site and developing custom modules.

As far as having to upgrade to Drupal 7 eventually, despite it being a valid point in deciding which version to use, who said you have to be running the latest version? As far as I know enterprises take longer than most to adopt new technologies.

Quite frankly, developing for Drupal 7 (specially large and mission-critical software) is a lot more expensive and complicated at the moment than using Drupal 6 and I would strongly advice against it for at least 3-6 months to wait and see how contrib modules will react and see how it will be adopted in general by the community.