Text won't be any less readable, Cleartype for desktop applications works exactly as it always has, regardless of how high a dpi display you have. It's only in Metro applications that they appear to have removed the sub-pixel rgb optimsations. Presumably that's because Metro tends to use larger fonts and large areas of flat colour, which reduces the need and benefits of doing so.
It's all academic in portrait mode anyway, since the orientation of the display pixels isn't suitable for Cleartype's RGB tricks anyway, so there is nothing to gain there.
Portrait is understandably a moot point.
Still text in article I read in the metro news reader, or the browser, etc. will be more jaggy than when read in the desktop counterpart (if there is a desktop counterpart -- if Microsoft is successful with metro there won't necessarily be a desktop app).