I had to go through that when I got my captivate.
Friday: Went to kiosk in the mall, told them I want the phone but ONLY if I keep my unlimited data from my Blackjack 2. He said sure, started activating the new phone, then told me I had to switch to the limited plan. After he spoke with customer care for an hour, he still said I wouldn't keep my plan with the new phone. I said forget it, I'll keep my black jack, just make sure I still have my unlimited, he said yeah sure.
Saturday: I open up my phone net to check my e-mail, and I get a message saying that I will be charged per minute for data usage.
OH. HELL. NO.
So I called customer service myself, they put me back on my unlimited plan. The customer service rep told me there was an issue with my black jack on the wrong plan, that I was getting unlimited data without actually paying for it

At this point I'm considering switching to Verizon, so the customer service rep told me to go back to an att store and speak with the manager, and have them call customer service, this way I'll get my captivate and stay on my unlimited plan.
Get to the store, and after about an hour waiting for the sales rep to talk customer care into giving me my plan, I finally got the unlimited with the captivate.
So yes, it is possible to still be added to the unlimited plan under special circumstances (if two idiots in a mall kiosk take you off the plan by mistake).