The main answer is "yes." However, ordering the phone through websites or going to big-box stores and the like could end up with your data plan being limited. A call to AT&T will fix it, but I went to an AT&T store to ensure that the process went smoothly.
I had an iPhone with the unlimited plan and had been out of contract since February. I used an Android phone from Craigslist with no problems until I bought the GSII in early October. The unlimited plan survived through it all, and we remain happily together.