drush 6.2.0 reports no version control found for core updates, but DOES find and use svn for module updates (pm-update) for drush 6.2.0, when running:

drush pm-updatecode --version-control=svn --svnsync --svncommit


on a drupal 7 site that only needs module updates, drush will properly commit the changes to svn as expected:

Project committed to Subversion successfully


however, if the site has a core update, drush will report:

No valid version control or backup engine found (the --version-control option was set to "svn").


and fail. does anyone have any idea why this is occurring, or what i should do to fix it?
I haven't used --version-control=svn for a while, and never liked --svncommit.  In any event, it looks like you have found a bug in Drush.  This should be reported in the Drush issue queue; however, since the current maintainers have pretty much moved on to git, I am not sure this code will continue to be supported unless a volunteer stands up to fix and maintain it.

In the meantime, you might try downloading Drush 5, and install it as a secondary version, perhaps setting up an alias 'drush5' to point to the older install, and use it only for core upgrades.  If that does not work, you could even try going back to Drush 4.  Note that neither Drush 5 nor Drush 4 are supported, though.