Fixed, sort of!
I'm having a very similar issue with iTunes 10.5, iOS 5 on an iPhone 4S. My regular (not smart) playlist was sorted by "Date Added" newest on top, and in my phone it seemed to be sorted by "Date Added" oldest on top (which is silly because I want to play my new music more often than my old music...)
Anyway, on a whim, I tried sorting my iTunes playlist by "Plays" and synced and it WORKED (properly sorted by "Plays" on my iPhone). Thinking I had fixed the issue, I again sorted by "Date Added" newest on top, and synced, and the iPhone reverted to "Date Added" oldest on top...
Thinking it might be just the "Date Added" column sorting that's F'd, I sorted by "Name," and synced and it WORKED (properly sorted by "Name" on my iPhone).
Not wanting to test each column, I decided to leave it sorted by "Plays," least on top, which is similar to "Date Added" newest on top, but not exactly. It'll have to do until Apple fixes this issue.
Conclusion: At least the "Date Added" column sorting in iTunes is completely disregarded on the iPhone, choose another sort column for now 😕