It was a slur; I'd argue that it's use as the respected mascot of a football team over the past 60 years has caused the word to transcend any derogatory meaning at this point in history.
Regardless, anyone who fakes "offense" at the term Washington Redskins is not being honest. The use of the word carries no more harmful intent than "Fighting Irish" or other non-offensive but commonly used stereotypes in today's society. Any evil that stems from the use of that word has long since died out.
The people who are looking to be offended by it are out just to get their names in the papers. There is no current regime of Native American prejudice occuring in this country, at least not anywhere around Washington DC.