Display.releaseContext does a wglMakeCurrent(NULL, NULL) internally, which is necessary if you want to make a context current in another thread. You can't have the same context current in 2 threads, you need to clear the current context in the first thread before making it current in the second.
With that said, if I understood correctly what you're doing, there's no gain from passing around a GL context between threads. You could achieve the same result if you do what Kai suggested (faster even, as there's no GL context switching involved). That is, load a texture/model/whatever in a background thread (the slow, IO related part), then synchronize with the main rendering thread and load the texture/VBO/etc from there. The Java-side synchronization cost remains the same, but you don't stall/flush the rendering pipeline by switching contexts, the driver is able to do other stuff while uploading your texture data to the GPU.
Anyway, the most efficient way to do multithreaded data loading requires 2 separate contexts with sharing enabled (see wglShareLists or the second parameter of the new wglCreateContextAttribsARB). Since we don't support multiple Displays in LWJGL, the only way to achieve this now is by using a Pbuffer. You create the Display in the main rendering thread, then you create a Pbuffer and pass Display.getDrawable() to the shared_drawable argument of the Pbuffer constructor. You call Pbuffer's makeCurrent in the background loader thread and start loading stuff. This way synchronization is done at the driver level basically, with most modern drivers supporting true multithreaded data uploads (the pipeline never stalls). You'll only have to do some basic synchronization for passing texture/VBO/etc IDs to the main rendering thread (that have been created in the Pbuffer thread).
I've also read that it is possible to create multiple contexts from the same device context. I'll investigate this and will try to add some new API to LWJGL that will allow the above scenario without the need of a dummy Pbuffer, so stay tuned.