Thanks for the info. That's crystal clear. It does sound like they've blocked data for that IMEI. Personally I don't care if they block data because I've already requested a data block. What I do care about is if they add back my data plan and start charging me for it.
It's a pity there's no way to change the IMEI of your iphone anymore. I'm not clear if that's because it's illegal in some countries or if there are technical reasons related to the baseband. I have a nokia that's out of service that could supply an unused IMEI and at least morally, I don't see any issue with this.
It might also have absolutely nothing to do with the IMEI. Rather, it might simply be that they're restricting which wireless access points names (APNs, think "proxy server") your account is permitted to connect to. The APN might contain different filters to restrict the speed and types of content that can pass through the network to your phone.
One APN might provide access to the "full" Internet (think ISP), whereas another APN might only provide access to the dumbed-down Internet (think WAP).
In theory, your $15 unlimited browsing plan doesn't need to provide access to the full Internet, but simply the dumbed-down WAP version of the Internet, so they might set up your account so that it can only reach the WAP APN, but not the ISP APN. Which would be just fine for your Blackjack because that's the access point it was likely pre-configured to connect to.
Your iPhone is probably configured to use the ISP APN, which AT&T might have registered as off-limits to your account.
You can test this theory (APN-based filtering instead of IMEI-based filtering) by reconfiguring your Blackjack to attempt to connect to AT&T's ISP APN (instead of the WAP APN), and see if you can still reach the Internet.
Or alternately reconfigure the iPhone (if it supports it) to attempt to connect through the WAP APN (instead of the ISP APN) and see if you can still browse the web. Note that this option, even if it works, might limit the range of websites you can visit, or even restrict what types of IP protocols (namely anything that's not HTTP) your phone can use.