This has been bugging me forever too! I never even considered "adviser" a possibility until I realized that my spellcheck didn't like "advisor". (And apparently neither does the spellcheck on SDN, haha -- my preferred version just got underlined in red.)
"Adviser" still looks totally wrong to me, and it must be because it's the more British way of spelling it, like that link from the previous poster says. Maybe we should petition Microsoft to add it the better version to their spellcheck.