STRING CONCATENATION NOCLASSDEF FOUND ERROR
In eclipse, if you switch to Java 1.5+, string concatenation will fail with a "NoClassDef Found Error". The only solution that I can find is to go back to Java 1.4.

ECLIPSE BLACKBERRY SIMULATOR/EMULATOR DOES NOT HAVE INTERNET CONNECTION
If your BlackBerry simulator can't connect to the internet, and you are using Eclipse, then you probably need to manually start the MDS emulator. This emulator will act as a BlackBerry server that your mobile device simulator can talk to. To start the MDS emulator, which is a .bat file on Windows, you just need to find it in your Eclipse BlackBerry Plugin path.

LINKS
How to alert a user from a background application
BlackBerry HelloWorld