That appears to be somewhat in the grey area. We had some people do it and they got letters from ATT. It may depend exactly which plan you have. It technically will work it is mostly a billing thing.

What is strange is this link a user who got the letter found

http://forums.wireless.att.com/t5/Apple-iPhone/Clarification-on-the-New-Smartphone-Data-Plans/td-p/1938734

It outright says you can do this and we have done it for other unlimited plans with no issues but blackberry seems to be something special. I would call and ask them, they make their money on the monthly subscription so they will not care a lot about the hardware you use. If I remember right the issue was more related to email than that data plan on the blackberry. I suspect it is just a matter of asking and getting it done the correct way.