I've had my inactive WoW account compromised as well. I definitely do not have malware, nor did I fall for any phishing scams. I suspect that some other insecure website I'd used the same email/password combination on was hacked (it's unfortunate they forced a switch to Battle.net accounts, because my previous account name was unique).
I was very unhappy with Blizzard's support actually - the GM I first spoke to said they would restore my items no problem, but the team the ticket was referred to first restored a few items to an alt and nothing to my main character. I complained, and they did restore some items to that character, but I still had a lot missing which they didn't reimburse, including Argent Tournament items of no monetary value that took a lot of daily quests to build up. I pointed this out, as well as how I was still missing a lot of gold. They not only refused to restore any more items, but basically accused me of lying about how much gold I had before ("we have no records of you ever having it"). I know I'm not wrong about that, because I was running addons tracking all my possessions which I could reference against, and I reported the account theft within a week of its occurring. I don't see why rolling everything back in the past week should be a problem?
Really has made me very disinclined to ever play WoW again.