Today I saw some entries in the review queue for new tag wiki's on the keywords Pex and Pipe. Both of these edits contained text copied from Wikipedia and no reference was given. Is this considered acceptable?
No it's not acceptable.
Firstly any text copied from any source must be a) referenced and b) allowed under that source's copyright.
Secondly the tag wikis are there to tell people how to use the tags on the site. They shouldn't be used for definitions of the term. We all (or at least we all should) know what a pipe is. What's important is what you use the tag for on the site.
From a copyright perspective, wikipedia is covered by a creative commons share alike license that requires attribution. Failure to provide attribution is copyright infringement and not something we should permit.