To narrow down exactly where the problem is, I'd recommend the following tests:
1. Try accessing your servlet from a browser on the BlackBerry. If the servlet is accessible from IE on a computer but not from the BlackBerry's browser, then it's probably a network-level problem. (For example: the mobile operator may have restrictions on accessing non-standard ports via HTTP).
2. If you can access the servlet from your BlackBerry's browser, then try a
test MIDlet -- with the same code -- but have it load another URL (one that you know should work like Google or JavaRanch). If you still get an exception, then the problem is the MIDlet code.