Motorola and Sony Ericsson phones return null for microedition.platform. Sanyo, Samsung, LG, and MIDP4Palm all return "j2me".
Most phones do not set a User-Agent header with MIDP HttpConnections, so you can't rely on that either.
When I've needed to identify the device, I've used a combination of microedition.platform, the canvas size, and the number of display colours. Unfortunately, it involves some guesswork and isn't very robust, but I've found it satisfactory for my needs.