No. It doesn't cost a red cent more. I got the messages after having my original iPhone unlimited plan (was using it on a Nexus One) spam me after I got an Atrix. I waltzed down to the ATT store and asked them what the deal was. I asked if this message was some sort of clever trick to switch me off of my unlimited data plan. The ATT guy chuckled and said no, all they needed to do was code my equipment as the Atrix in their system and I would of course retain my unlimited data.
It was a bit shady and I do still feel that now that ATT knows my IMEI and knows I have unlimited data on a "4g"

phone, they will emply yet another trick to ween me off of my unlimited plan.
There are always more provider choices if they want to play games. So far, so good...for now.