I am not able to reproduce the problem, I can add spry validation to a data assist form without any errors.
are you adding a new spry text filed to the data assist page? or are you replacing an existing form element?
I assume you have deleted the Dreamweaver cache file?
can you send a copy of the page you are having the problem on so I can examine the code? it could be something specific to the form element / table column names.