Why does my Samsung Galaxy S lose data for some applications?
It may actually be a Samsung problem.  Here's a similar problem report (app on Samsung becomes forgetful) with DropBox: http://forums.dropbox.com/topic.php?id=37207

The DropBox folks claim they fixed it (though perhaps they just worked around it).  I couldn't find any details, so it may not be the same underlying mechanism as the apps you're having problems with.

Assuming the apps in question store the login data in an sqlite database internally, here's a SO question about a similar problem:
https://stackoverflow.com/questions/5886019/android-application-samsung-galaxy-tab-2-2-does-not-sometimes-write-to-the.  Sadly, it is currently unanswered.

The Samsung developer website does not have any mention of sqlite or persistence problems that I could find.  (But there isn't a whole lot there.)