Blogging platform for enterprise?

[trigger_my_passion]

One of my colleagues asked me this:

........ has kindly mentioned that you may be able to provide some advice and perhaps a list of options for the selection of a blogging system. Here are the high-level requirements:

* LDAP support

* easy to use for those not too savvy

* "brand-able"... able to plug into existing portal and support css

* multiple user roles

* extensible blog entry form... need to add custom fields and support application integration

* support a community size of about 1,000 bloggers (maybe more) and > 500,000 readers

* highly scalable... just add hardware

* and the other basic blogging functions

I am currently evaluating all options: open source, commercial, in-house, and hosted. I am putting together a best-of-the-best list and plan on building a scorecard around this for presentation to upper management.

Can anyone recommend me possible blogging systems that meet the above requirements?