It's more that now legitimate businesses can do these things without fear of legal action. It's legal now for Google to post a custom app on the web that unlocks your iPhone and installs Google Voice. It's legal for T-Mobile to unlock your iPhone for you.
These things have existed on the grey market for a while, but legitimate businesses can't trade in such things. Now they can.
Curious as to why this wasn't possible before, seeing as you could buy an unlocked phone from Apple?
Edit: I think I'm confusing jail breaking with just untying a phone to a carrier. You can, of course, buy phones untied to a carrier already from Apple. However, you must still install apps via the App Store.
Apple's not going to sue itself. So they can of course unlock their own phones (barring a contractual restriction with AT&T or similar).
The change means that, i.e., Google, or TMobile, or some other legitimate competitor to Apple could use jailbreaking or unlocking as a competitive strategy in a way that Apple wouldn't like. For instance, Google could operate their own app store, and consumers could use Google software to unlock their device and install Google's app store.
While Apple does offer unlocked phones in some countries, they typically don't do so willingly. All of the unlocked phones that I am aware of come from countries that forbid locked phones to be sold.