For IPB customers, you could setup a Subversion repository and we could run ' svn update ' each night to automatically secure our forums better by upgrading them to the latest patched versions.

I don't think this will happen, the reason being it would be to hard to control who has acesss to the subversion repository. Anyways if IPS can't take an hour and post security patches, then we are in trouble, but the do so we are not in trouble.

I don't see the IPB 2.1.6 branch being modified very often, and if it is, then I see a release being made.

does subversion allow automerging of files? (so that mods keep working after you update your ipb board)
if it would then adding a subversion possibility for customers would be interesting and something for which I'd happily pay extra.

There is not such thing has 'auto-merging", the concept is just not possible. Sub-Version does allow you to compare, and merge by hand, changes from the last revision.

If you need a sub-version repository, there are tons of paid and free services you could use.

I know that cvs has automerge (you can work with several developers on the same source files, on commit the changes are merged into the file)...

subversion is also a version control system, so it should have something similar like that right?

SVN has a system where, if developers commit different versions of the same file, it adds the different revision's files to your source tree.

So, if you had a file called "myfile.php" and two developers committed two different sets of changes, one at revision 100 and the other at revision 101, you'd get the following:

myfile.php
myfile.php.r100
myfile.php.r101

You'd then be expected to run a diff on the files, work out the changes, fix up myfile.php and run the RESOLVE command on that file.

Sub-version has a similar feature, but its not "automatic" so its called "auto-merge" its called merge or resolving conflicts.

sub-version is CVS 2.0 basicly, created by the same developers that created CVS