D7 as multisite or as a library?

Hi:
If you need multisites with differents versions (D6 and D7) I suggest you install two multisites : one with D6 core and the other with D7.
The goal of multisites is to share the same code (D6 and D7 don't). Perhaps you knows that a module could be only in a version. That would be a restriction too.
I hope these comments help you.

Manuel.

Wow.. Obviously you have a strong hat for the "library" approach.. Can you elaborate on the complication it causes other than the obvious management of the symlinks?

Well, as I touched on, you're going to confuse anyone who comes along to work on the site and is expecting to encounter a normal Drupal multi-site installation. You're also just so outside of the norm of what Drupal expects itself to do and what others are going to be expecting Drupal to do that, if you run into problems in the future, solutions others will offer you may not work just because your system is so twisted beyond the norm.

The Library approach is something I picked up from a presentation I came across online from Warner Bros and its the way they manage their Drupal sites..

Could you link to said presentation? I'm curious to see what their justification for doing this might be.

How do you manage upgrading individual sites to a new core version rather than the all of them?

I'm assuming you mean major core version upgrades (like D6 to D7) as opposed to minor point releases, which should be a non-issue. Assuming that, it's fairly simple. Kind of annoying and maybe confusing if you're not used to configuring web servers, but not too difficult and not something you'll have to deal with often. Here's my approach when I need to do something like this.

First, just install the new Drupal core in a separate directory from the old one. Once you have an upgraded version of a sub-site which is ready to go live on the new core, alter your web server's configuration so that it uses the new core directory as its web root instead of the old one when that site's domain names are accessed. Let Drupal take it over from there.

As an example, if you have a bunch of D7 sites and D8 has just come out, your file system would look something like this:

/path/to/webfiles/d7/
/path/to/webfiles/d8/

Now if you want to go live with a D8 version of a site at http://example.com/ , but want everything else to still use D7, your web server daemon's config file would look something like this:

if (host == 'example.com') {
  web_root_directory = /path/to/webfiles/d8/
}
else {
  web_root_directory = /path/to/webfiles/d7/
}

Obviously, the specifics of this will vary depending on what web server software you use, what level of control you have over your server, and so on. But it should be fairly simple. Any sort of redirection, as Lakeside puzzlingly mentions (can't see what they're getting at), shouldn't be necessary.