Well, actually installing the files shouldn't be too hard (I also had a look at \"unshield\"), but I actually don't think we can get it to work with every platform (a program which uses the same source code for all platforms won't be possible).
As I see, all you need to locate on a Windows D3 CD is the \"data1.cab\" file, extract it, and then copy the rest of the needed files from CD1 & 2 to the target directory. After that comes patching ... etc.
As of now I could try writing such a program, but I only have Win XP on a virtual machine and I am not sure if that will run correctly on Vista/7.
For Linux: the installer is still working and there's also a Mercenary installer by Loki, but it's all a bit difficult to use - writing an installer should be easier than for Windows, so I could do that as well.
For Mac: I don't have any Macs here and I have absolutely no knowledge about it.
If I should do anything, I would use C++, using Qt4 for the GUI, because it's Linux/Mac/Windows compatible.
EDIT: I've made an installation program for Linux now and it was really easy. Everything's working except for Mercenary and patching, which will be added when 1.5 is ready. The next days I will port the program to Windows. Screenshot from Linux is here:
http://intelcentino.in.funpic.de/upload/d3installer.png (but note, this is not final)