It wasn't Arcus himself, he was RATted and in turn his website was. There wasn't a breach of defenses of the site, it was simply Arcus being infected by something malicious. Here's more info on it:
MSN's, Skypes, Gmails, Sythe accounts, Playerauction accounts - Anything can be hacked. Websites are probably still the safest option. It's a risk that you will have to take anytime you purchase gold. Just take your time, if something doesn't feel right ask for a Private Message on Sythe or just don't buy any (Or large amounts) of gold until you feel comfortable.