Regardless of what it says about the merits of id's latest creepy corridor crawler, it is possible to play Doom 1 inside Doom 3. Why, you ask? It's not because Doom 3 is so dark, you'd rather play Doom 1 in the glow of a terminal screen; rather, the Terminal Doom project is a proof of concept study of "fully interactive surfaces." To that end, the mod creator writes, "In more than one respect, Terminal Doom itself is a dead end."
Awww, but that doesn't mean you can't try it out. All you'll need is the retail version of Doom 3, either the shareware or retail version of Doom 1, and some patches and you're set to go. The in game arcade cabinets are pretty great also.