Is there a UI toolkit for Java ME that I can use to generate UI elements on the fly, based on, for example, JSON or XML that I send to the device by way of a web service pull?
Multiple UI frameworks available for Java-me application. But i suggest LWUIT framework is good. LWUIT offers advanced UI capabilities and a clean API that is inspired by Swing. You can develop your own theme and animated screens by using LWUIT ResourceEdit. For more info see this LWUIT blog.
You can download latest LWUIT from this repository or you can use latest release of LWUIT version 1.4 available. LWUIT supports Blackberry, Android and Symbian OS.
For more info see this links for your reference,