The Oppo Find 7 phablet has been rumored before to come with a 2.5k resolution (2560×1440), but now it has been officially confirmed that it will have a 5.7" screen with a 2560×1440 resolution, which resolves to 515 PPI. Hopefully Oppo can increase the battery size, too, to make sure it has battery life that is at least as good as it would've been with a 1080p resolution.
This is still my biggest worry about the 2.5k resolution on small devices. The panels keep getting more efficient every year, but even if they get twice as efficient in order to completely cancel out the power draw of a display with twice the pixels, I still can't help but think that I'd rather get the panel with last year's resolution (which is good enough) but with this year's display technology, in order to actually get a display that uses half the power as the one from last year.
For this reason, I'm not a big fan of phones with 2.5k resolutions, because even 5" smartphones are constrained by the space they have for battery, and also by costs. However, I feel more positively about 8" tablets starting to use this resolution, because for example if 8"-8.3" is going to be the norm for Android tablets next year, then the 2.5k resolution makes more sense for them, so they can pass that 300 PPI mark, because with a resolution of 1920×1200 they are under it.
The same goes for 10"-12" tablets, which should remain 2.5k in 2014, but I expect them to move to 4k resolution in 2015. Unless you want super-precise drawing on a Note-like tablet with such a resolution, though you may want to wait a year or two longer before getting one, because higher resolutions are not just a major setback for power draw in screens, but also a major setback for GPUs and gaming. You may have a GPU in 2015 that is four times faster than the one in 2013, but the resolution would also have four times more pixels, so the graphics of the game shouldn't be much better in 2015 on such a tablet, than the graphics you're seeing this year on 1080p devices.
Still, it seems higher resolutions in mobile devices are inevitable, and we can only hope the OEM's will eventually stop raising the resolution unless it's actually beneficial to the user and not just an easy way to market a new tablet, and start focusing on other new hardware innovations. Otherwise, they may not stop until we get 8k smartphones.