As one of the previous posts describes, heterochromatin is not always cold and inert. It goes through dynamic transcription during cell cycle progression. The fact that all of the earlier studies used mixed population of cells is the reason of the controversial heterochromatin transcription.
To expand this a little bit, in fact most of modern molecular and cellular biology researches (except cell cycle studies) are done using cells staged at different cell cycle phase. If you think about it, the cell cycle approach could be applied to most of these researches. It is not unexpected if a lot of our current knowledge is refined by taking this approach, for example cancer formation and development. A new science direction is approaching.