OMG -- they actually post information regarding this query on Continental.com of all places!
http://www.continental.com/web/en-US...t/showers.aspx
So yes, there will be access to showers on arrival in ARN. However, at many airports these arrival facilities are at nearby hotels, which require a 5-10 minute shuttle ride, so they may not be all that convenient. Remember, there aren't a lot of arrivals lounges in Europe - they have to be built outside of customs and thus cannot be shared with departure lounges, so you really only find them at a large carrier's hub airport (e.g., AF's arrivals lounge at CDG).
If you private message
CO Insider he should be able to give you more color on the exact facilities at ARN. I believe they change from time to time and sometimes are different depending on mid-week or weekend, so they don't post the full detail online.