It looks like MediaWiki folks revealed a big secret - they DO allow tagging (and branching) of extensions.
See this thread: http://lists.wikimedia.org/pipermail/wikitech-l/2008-May/037722.html
I was misled by "Don't touch the release tags at all. Ever!" on http://www.mediawiki.org/wiki/Commit_access and I have a feeling that you were too ;)
Having this in mind, I think it'll be reasonable to have SMW code tagged for each release - this will help a lot with rolling back installation and stuff.
There could be tags like this: http://svn.wikimedia.org/viewvc/mediawiki/tags/extensions/SemanticMediaWiki/REL_1_1_1/
All you need to do is to run:
svn copy . http://svn.wikimedia.org/svnroot/mediawiki/tags/extensions/SemanticMediaWiki/REL_1_1_1/
every time you do the release.
Hope it makes sense.