If I understood correctly what Hannes said, the upgrade from 1.5 to 1.6
cannot be made the same way as we will do it, for example, from 1.5.14 to
1.5.15, by only copying the new files over the old ones. Not only the files
will change but also the structure of the tables, and some files will be
deleted. Those things don't happen in a regular upgrade.
Nevertheles, it won't be necessary a migration script like we had from 1.0
to 1.5. No need for a new 1.6 site to migrate the old 1.5 site's database.
We will need a script that will take care of the changes in the tables and
also of deleting the files that will need to be deleted.
As for the 3PDs, they can offer their components packed with the "upgrade"
method in the xml file and also provide scripts in the install process to
take care of the changes in the tables structures.
An active upgrade script would be the best option (for J not 3PDs). This
script would be installed as a component in the 1.5 site and then, when
started, would take care of the database changes, of deleting the unecessary
files and also of fetching the new files directly from Joomla 1.6 repository
and placing them in their due places.
The limting factor in such a script, as Hannes said, are the
max_execution_time limitations and the size such a script would have to
handle all that stuff.
I'm sure someone will come up with a nice idea for that script that will
make everybody's life easier.