The company made the announcement from its Tokyo HQ, with the final wrap-up coming considerably earlier than the mid-year prediction we've heard in the past.
That said, there may be internal elements of the takeover that still have to be ironed through, and existing handsets might have to wait for a while until updates appear for related on-board software that refer to the SE brand (if Sony is actually even bothered with it). But that aside, the deal has been concluded and Sony Ericsson is no more.
Sony now plans to “further integrate the mobile phone business as a vital element of its electronics business, with the aim of accelerating convergence between Sony's lineup of network enabled consumer electronics products, including smartphones, tablets, TVs and PCs.”
Considerable progress towards this goal has already been made, and in fact was underway well before talk of an Ericsson buyout even began to surface, with Sony-specific names like Bravia and Exmor appearing with increasing regularity on mobile handsets.