Answer:
For some people, yes. However, this is more a matter of intolerance than any real problem with atheism. In general, atheism (and by extension atheists) are no more offensive than a member of any religious or cultural group.
--------------------------------------------------------------------------------------------------
Intolerance and offense has no passeport, no faith, no color and no sex.
People are offensive, a idea is offensive only used by offensive people.
Some atheists hoist the flag of atheism in order to attack faith in a God. This is offensive. And ironicaly, can be seen as a religion.
Because the consequence of that is lack of respect. It is not a "scientifical" or "theological" debate about the existence of god anymore. It just becomes a "who's gonna be the more stupid" contest, just because both sides absolutely want to be right !
So, the anwer is Atheism is not offensive, but an atheist can be. And you can replace Atheism by any conceptual common noun.
I considere myself as a atheist : I never discuss someone's faith. Everyone is free to believe in whatever he wants. Some people do great things guided by their faith. And I prefer to share with a sincere and respectful believer than with a "militant" atheist whose only goal is to destroy faith.
As far as I'm concerned, i think I don't need to believe in a God.
May be I'm right, may be not.
After all, is being right on that point so important ? isn't it better to be happy ?
I try to be humble, kind and respectful, and the minumun I expect from the other is this reciprocity.