It's been topic for discussion for a while now. Personally I think we'll see some soft of announcement in April during the GTC, and later on a broad announcement in the Computex timeframe. Likely, and here's why the post starts with 'rumor' only the mobile parts will be announced and launched first, and not the desktop graphics card parts.

According to sources in the industry the mobile replacements for GM204 and GM206 are planned to be released first. Pascal is Nvidia's new GPU architecture, fabbed at 16nm. Everybody is waiting on big Pascal with its 17 billion transistors. As stated we expect Nvidia to share more details on the GPU Technology Conference as the CEO of Nvidia is holding a keynote speech.
 

Initially everybody expected an April/May release for desktop GPUs based on pascal. 

The 'Big pascal' that the PC gaming community is waiting on is called GP100 GPU based on a TSMC 16nm process fab. We'll call that GPU Big Pascal with billions of transistors (16 to 17 billion rumored). Two things can happen here, it will have HBM2 which is complicated, or the cards will use GDDR5X.

In a latest timeframe prognosis Nvidia are rumored to launch the first high-end GP104 GPUs with models GTX 1080 and GTX 1070 (should Nvidia use their old naming scheme). These chips would  come in a 37.5×37.5mm BGA package, and has 2152 pins. Pascal GPUs are fabbed in the Taiwan Semiconductor Manufacturing Company (TSMC) (recently hit by an earthquake) based on the new 16nm FinFET process, a process that uses the popular stacking method that NAND these days uses as well. It should result in significant power savings. 

Anyway, so be it, mobile parts will likely be released first.