There are a few threads here and on XDA that address this. Basically, HTC made it so that when you move an app to the SD card, the accompanying lib files go with it. For some apps, this frees up a ton of space in /data/data. For many users, this should help alleviate the problem. The problem isn't gone, it's just that they raised the ceiling on it. You'll still eventually hit it.
From what I understand, some of the worst offenders that cause this issue are:
-HTC Sense FB contact sync
-HTC Mail
-Netflix
If you used all three of those extensively, you probably ran into this issue at some point.