I have MT4.3 Pro with a 5 author licence. I would like to create a community blog i.e. a blog where any authorised member of the community can post an article on a single community blog. Is this possible under the terms of the licence or am I restricted to just 5 authors (i.e. a "community" of just 5)?
In addition to the community blog, I intend to have 2 or 3 classic (single author) blogs. Does my licence allow me to do this? If not, could I do it with the Open Source option?

You should go to movabletype.com and contact their sales support. Then, if you're feeling up to it, post their response here so that it's part of the forums. This is really the sort of question you want an official opinion on.
I'd like to know this too - did you get an answer, cfish?
Offical license seems to say 'no' -
...
1.3 For the purposes of this Agreement, "User" means a unique login name generated or recognized by an instance of the Software that possesses the permission to do anything more than commenting on one or more blogs and which has been used to log in to the Software within the prior ninety (90) days.
...
Therefore, blogging > commenting and a blogger would count as user not profile...
I've contacted MT to confirm either way...
"This answer for Cfish comes from Sales:
The 5-Author license comes with 5,000 Profiles. Cfish, the licensee, can make available up to 5k profiles for visitors to create and post on the community blog.
However, an "author" is anyone with a unique login to Movable Type; for example, anyone with permission to change blog entry content or modify blog settings.
Licenses are based on active 'authors" who have logged into the system in the past 90 days. Site visitors, readers and commenters are not considered "authors" for the purposes of licensing.
A profile is an online identity that includes a photograph, personal bio information, links and other details about you. It enables visitors to learn more about each other and to make connections and build trust on the blog. You may choose to limit commenting to people who have created a profile.
Also, this MT 4.3 Pro 5-Author license allows Cfish to have unlimited blogs."
If you have any further questions on these issues, please let us know.
Yep, that's right. "5 Authors" means just that - not really a "community" blog in my opinion. That's why I am now using Wordpress MU and am kicking myself for buying a license for MT. You live and learn.
Just to clarify some key items that Olly asked about:
If a "personal blog" is created for a user when they register, then the "user" becomes an "author" and the larger "blog owner" has "spent" one of his 5 authors.
Because MT grants the user full admin rights upon granting a personal blog, a "user" becomes an "author."
As such, if you grant your users personal blogs, those users will count as authors, and you will be subject to the limits of your MT license.
o
If you do not grant your users blogs, you have a limit of profile users whatever the number of profiles your license allows.
Thanks
Correction: dropped a word in the last sentence. Should read:
If you do not grant your users blogs, you have a limit of profile users, that is limited to whatever the number of profiles your license allows.