Blizzard will soon be making it mandatory for everyone to have a merged battle.net account to play WoW (and other Blizzard online games). For the EU it seems that after November 12th you will be unable to log on with a "regular" account, and I expect US accounts won't differ much in terms of time frame. So if you haven't already, you should consider merging your accounts before the "rush" that's bound to occurr when this does become mandatory next month.
The wife saw the notification when she went to log in, so I went ahead and merged my WoW and Battle.net accounts before I logged in. Pretty painless.
I half expected this after hearing the rumors about warcraft 4. I have not had a battlenet account in years.