I don't think it's been properly explained what's going on here. So let me try to clear things up.

First, the issue was not instigated by Joomla FCK, even though a simple space character like   (that Joomla FCK Editor creates) will delete all the content after it.

Second, until FCK editor catches up with the changes in Joomla 1.5.8 you will need to do the following:

 

1. Login to your /administrator/ control panel.
2. Under the "Content" menu, click the dropdown item "Article Manager"
3. In the Article Manager menu, click "Parameters"
4. In the new window scroll down until you see Filters, select the "Whitelist" radio button, next select "Super Administrator" or as many groups you want to prevent having this problem.
 

NOTE: Don't select more than you need to "super administrator" will probably be fine for most people.

And that's all there is too it! This was added as a security feature in Joomla 1.5.8. While it's obviouslly an annoyance, it adds extra security to Joomla. You can read more about this update here.