There are changes to the way the Common Navigator selects content extensions for label providers that might be incompatible with existing implementations. These are in bug 252293 and are available in any I build after I20090125-2000. So far so good. I have tested these with JDT, CDT, and the Eclipse SDK (without JDT) and everything seems OK.
We are waiting on a version of WTP with the ICU changes to test with that.
The reason for these changes is that the previous behaviour was non-deterministic (based on the presence of something in a cache), and that in order to fix the above mentioned bug there is a possibility of introducing incompatible behaviour; we wanted to get this into M5 to maximize its exposure.
If you have code that uses the Common Navigator, please test it in the next couple of days (say by Weds 28 Jan) if there is a possibility of incompatibility in the way you use label providers. See the comments in the above mentioned bug for more details.
If you do have compatibility problems, please immediately open a bug to Platform UI and we will get it addressed this week in 3.5M5, provided we get the bug report by Weds.