Should co-author mention you in talk-abstract
I would say it's certainly inconsiderate, but I don't know if I'd go so far as to say "unethical", as long as you are credited in the talk itself.

Rather than telling your coauthor it's "unacceptable" to leave it out, he would be able to save face if you treat it as a simple oversight, whether or not you really believe that it is.  (Even careful people make mistakes, and I am always in favor of applying Hanlon's razor.)


  Hey, I saw your abstract for conference XXX.  It looks like a very nice venue to discuss our work.  One thing: it looks from the other abstracts like this conference follows the usual convention of mentioning coauthors in the abstract ("this is joint work with...") and it looks like this is missing from yours.  If it isn't too much trouble, would you still be able to add it?  I would really appreciate it.