Hmm, I was planning on going with C# but I've decided against it. Libraries for C++ are better documented.

That idea about duplicating lights is good enough if you plan on only connecting the pieces through basic empty corridors, otherwise some glitches would persist. For instance, imagine a light reaches the portal from one side but is partially occluded. If you only duplicate the light on the other side, you get no shadow from the occluder, but you do get the shadow on the side which actually owns the light. Thus, the shadow would be clipped on the portal. Also, determining the subset of lights that potentially reach the portal doesn't seem like an easy problem.
The only true solution I can think of would be to temporarily stitch the adjacent pieces and calculate the lightmap as if it were one piece. Seems like there would be plenty of redundant calculations though. There's probably a way to optimize that without having to stitch every single piece together at once.

Do you have any other contact method, besides ICQ? I guess I could get Trillian or something if not.

Sorry to invoke Half-life again, but have you thought of using a similar "buffer zone" method, where two maps overlap to a certain extent and it starts loading the next one as you enter the zone? I'm thinking of how it was possible to have enemies chase you straight from one stage to the next, right through the "Loading..." message. And when you double back a little bit it doesn't suddenly load the previous level when you cross the line, you have to go far enough back to exit the buffer zone again.

I know you want a standalone game that you can own, but if it doesn't work out, you could always develop a Half-life 2 or Doom 3 mod. They've released special free versions of Softimage and 3DS Max specifically to make modding them easier.

Buffer zones are great if you want to load between levels. I think heeen wants to load dynamically and connect the pieces with portals, so that wouldn't work with buffer zones.
Also, Half-Life's engine isn't good for neither heeen's nor my project. Half-Life isn't an engine designed for MMORPGs, and it's not the best solution for my sidescroller (a bit overkill in some things and a bit restrictive in others). I just want to make something by myself so I can gain experience, and also to prove to myself that I can do such a thing. Ownership is the least of my concerns, but I would like to keep the doors open for commercial exploitation in case something good comes out of it.
Nevertheless, thanks for the comments.

Awesomeness! X3 Imagine an actual BLAME! VG that Nihei intended just as in the Manga... Whoa. Well, whith out the Virtual Reality--ness. XD