Mediawiki 1.20 stable is expected to come out this month (Nov.) and we are currently testing the 1.20alpha version. It works ok with all non-SMW extensions used on our site (biowikifarm.net) as well as with SemanticMediaWiki, SemanticResultFormats, and SemanticDrilldown in versions 1.7.1, 1.7.1, 1.2.2 respectively.
However, as soon as SemanticForms in version 2.5.1 (or SemanticInternalObjects in version 0.6.9) are added, both normal pages and Special:Version fail.
For SemanticInternalObjects the error is:
Fatal error: Class 'SMWSQLStore2' not found in .../mediawiki20/extensions/SemanticInternalObjects/SemanticInternalObjects_body.php on line 56
(I have also submitted a bug report for the similar bug in SemanticForms: Bug 41640)
PS: The extension SemanticInternalObjects can not be selected in wikimedia bugzilla, although it is maintained in WMF git.