It's not very hard, I've got a build that has something like this, but the impact in-game is terrible, the game drops to 1 FPS when lots of lights are displayed, I'm not sure if the way I got it implemented would be part of the problem thought <!-- s:) --><img src="{SMILIES_PATH}/icon_smile.gif" alt=":)" title="Smile" /><!-- s:) -->.
I remember I talked about this with Janis time ago and he gave me reasons of why it impacted the performance, so until a better way for making REAL dynamic lights is found, I guess possibilities for real shadows aren't much <!-- s:( --><img src="{SMILIES_PATH}/icon_sad.gif" alt=":(" title="Sad" /><!-- s:( -->.