I read on this page http://www.homelanfed.com/index.php?id=25316 that id software will release the doom 3 source code along with the sdk. Does anyone know will we be able to use doom's script engine from the source or will it be illegal unless we buy a licence?

When they say source code they don't mean the engine. They're talking about the game specific code stuff (e.g. weapons code, AI behaviour code etc) and thus will not include the scripting engine (as it will be part of the engine source). What is it that attracted you to D3's scripting language? It is likely that another scripting language exists that has what you want.

Well, currently the most similar script engine to doom's is AngelScript, but it doesn't support inheritance like doom's.