I've had this happen a few times since Lr3.4RC (or at least I don't recall it happening with previous versions) - I thought it was me, but this time all I did was click a thumbnail and it switched from portrait to landscape orientation.
I realize, when people make feature requests, they are interested in native functionality. People sometimes offer work-arounds in case they help get by in the mean time. Hopefully it's understood that the intent is not to take anything away from the original feature request.
With that in mind, Exportant also has an option for PNG export which may be relatively convenient.
I've been wanting this feature for quite some time.
Not sure why it hasn't been implemented yet, as it would be very simple to do.
If we're able to use so many powerful features within Lightroom (like clone/heal, etc.) without needing to export to Photoshop; why not enable a simple PNG export option?