You can use a nurbs sphere (use a surface shader ambient color as the image) and uncheck primary visibility, but leave on 'visible in reflections'.
There are a number of ways to do this though, and they all depend on the contents of the scene and preparation. For example, some of the best reflections I've obtained were by using the MIP shaders and this guide:
http://www.dmmultimedia.com/3dtips\_11.htm
it's more of a setup but the results are very photorealistic. I'm sure there are more simple ways though, maybe someone else will provide a solution.