I have no clue how or why exactly, but when I upgraded the (otherwise really great) Link-summarizer plugin from 1.8 to 1.8.1 suddenly the post contents of some posts completely disappeared from display... Almost as if the new version messes with the_content in some way. Strange. I tried to re-download and upload manually to be sure there wasn't a file corruption issue, but nope. Who knows - could be some interaction with another plugin, but I've never had any issue with Link-summarizer before (well, outside from figuring out how to use regex properly so as to exclude especially the flash-based embedded links, but that's of course up to each user!)
I rolled back to 1.8 and that at least solved the issue for me.