There have been a number of problems since the upgrade, backing down to 1.7 should fix those problems.
Upgrade bundled Prototype to version 1.7.1
Integrated in tapestry-trunk-freestyle #977 (See https://builds.apache.org/job/tapestry-trunk-freestyle/977/)
TAP5-2009: Downgrade bundled Prototype version back to 1.7 (Revision 0eaeb8bb5e70dfe15e486e6b3e734c836b3f2734)
Result = FAILURE
Would it be a viable option to identify those problems and fix them? The only one I know about is the 'DateField error' which can be easily fixed in prototype.js. Shall I submit a patch?