A quick and easy method to read an InputStream to a byte array is available in the IOUtilities class of the BlackBerry API.  I didn’t check if this is available pre 4.6 or not, but it does exist in 4.6

To read an InputStream directly to a byte array you can do something like this:

FileConnector fconn = (FileConnector)Connector.open("testfile.txt");
InputStream inStream = fconn.openInputStream();
byte[] byteData = IOUtilities.streamToBytes(inStream);