From the blog post:
Hello everyone. When we kicked off the global rollout of Windows Phone 7.5, I told you we planned to balance quality with speed. And for three weeks we’ve been watching quality while sending update notices to a growing fraction of phones.
It’s gone well. So today we’re fully opening the spigot —slightly ahead of schedule—and making Mango available to nearly everyone in the current delivery pool (see Where’s My Phone Update? for the details).
In addition, Microsoft also trailed a second, carrier-specific update:
One last thing. Next week we’ll start delivering some model-specific “firmware” from handset makers, so don’t be alarmed if you receive an additional update notification on your phone. These updates are designed to activate new Windows Phone 7.5 features or improve your phone’s performance. Not everybody will receive or require one: It all depends on your country, carrier, and phone model.