Read this. Be very careful about using Singletons as a simple substitute for global variables; they're absent from the language for a reason. It's your program though, I don't have to maintain it; so do what you want.
The double check locking pattern fails for the RIM VM since the write orders are not guaranteed so if you use singletons the best thing you can do is make sure you instantiate them immediately in series before launching any additional threads.