28nm Maxwell chips are currently available in the GTX 750, 750 Ti, but 20nm designs might be delayed.
Sweclockers is reporting that Nvidia's next GPU design will be delayed from late 2014 to some point in 2015.
The rumor is coming from Taiwanese contacts claiming that TSMC, the foundry that produces chips for Nvidia, is not prepared for a full-on 20nm chip production run.
Currently, Nvidia relies on two major designs in its GeForce graphics cards. Kepler is the chip type found in many of the 700 series GPUs, as well as the Titan line. A newer architecture, called Maxwell, was introduced in the GTX 750 and 750 Ti back in February. These existing Maxwell chips, which are also at the heart of the GTX 800M chips making the rounds in new gaming laptops, are built on a 28nm fabrication, while the Maxwell's of the future are going to be built with the aforementioned 20nm process.
The reported delay means the next GeForce line -- presumably the GTX 800 or GTX 900 series -- will be bumped from Q4 2014 into 2015. Sweclockers says there's a chance of one high-end 20nm Maxwell model being released in 2014 (that line's equivalent of the GTX 780 ot 780 Ti, perhaps), but there's no guarantee of that happening.
The above roadmap (larger version here), which was shown at the recent Nvidia GPU Technology Conference, shows Maxwell on the edge of 2014. A Q4 2014 launch would keep the next major GeForce in-step with the typical 15- to 18-month GPU cycle, as the GTX 700 series was launched back in May 2013. A delay into Q1 2015 would bump the 20nm Maxwell off this path, but performance from the current line of GTX 700 series GPUs is strong enough to keep a (hopefully) short delay from being a disaster.