Phone
Samsung has confirmed, at the Semiconductor and Display Technology Roadmap Seminar, that it is working on a smartphone with a QHD (2560 x 1440) AMOLED display, according to the Korean media. It's not a confirmation we needed since it's fairly certain at least one of the Galaxy S5 models will sport a QHD (2K) display, but for those that were still skeptical, this should be enough of an indication that the Galaxy S5 will feature one of the most pixel-dense screens out there upon launch.
Samsung apparently also confirmed that it plans on launching smartphones with UHD (4K) displays with a resolution of 3480 x 2160 in the future, which would result in a pixel density of more than 700 ppi, making it suitable for phablets. No time frame was offered, but it's likely we won't see these UHD displays make it to phones any time soon. Samsung basically said that UHD displays will come to smartphones somewhere down the line, so it probably won't be before 2015 that UHD phone displays become the next big thing.
