CoolTechZone has posted a technology primer on High Dynamic Range (HDR) technology.  HDR is featured prominently in Valve's Lost Coast level for HL2:

Using HDR, the developers don't have to use hidden lighting and can render lighting completely through the radiosity method. The advantage that this would give them is that they can achieve even more realism as radiosity will allow them to control light's behavior in such a manner that they would be able to achieve effects such as change in lights characteristics (mostly color) when it passes through colored glass tiles and such.