I have been multiboxing since BC and have never gotten a ban for that. Even though people whisper me all the time saying they reported me.

But I was banned before on a single account. I had a team of 5 shamans and wanted to replace one with a warrior tank. So I was power leveling a warrior by himself over a weekend and received a 72 hour ban. I believe because I played about 44 out of 48 hours over a single weekend.