Sometimes I like to play around with the registry to see what little tid-bits of info I can find in it. Here are some interesting entries I've found recently (and not so recently). It goes without saying that modifying registry entries is very dangerous, and you can brick your device doing it.
Cellular control panel (settings item) --> show/hide the 3G toggle option.
Make the status bar on top show "4G" for an HSDPA connection. It can show other text, too. There are also entries for 1XUMTS, 1XEVDO, LTE, etc.
[HKEY_LOCAL_MACHINE\Security\Internet Explorer\Custom Headers\HTTP]
This is a custom HTTP header inserted by AT&T devices whenever you browse the web. I don't like the idea of telling every site what device I'm using, so I deleted the entry.
[HKEY_LOCAL_MACHINE\Software\Microsoft\WISPrClient\1.0\ATT\Hotspots\attwifi]WISPr allows phones to connect to certain pre-programmed hotspots without going through manual (browser-based) authentication.