There are some existing LEGO CAD's available. The most well-known examples are LeoCAD and MLCAD. MLCAD is the most popular, but lacks Linux support (so you have to use WINE to run it under Linux). LeoCAD is the only editor that supports Linux natively, but has poor usability IMHO. Klotz is a Free software alternative with good usability and multi-platform support.
The first beta of Klotz will be released within this month with a (near-) complete model editing feature.
Longer term goals for Klotz include:
- Implement interoperability with other 3D modeling software (such as Blender)
- More rendering backend support including Yaf(a)ray
- Export to vector graphics
- KParts implementation to browse LDraw models in the web browser on-the-fly (which I promised at aKademy 2006)
- Full Mac OS X and Windows support (Win32 build partially works for now)
- World domination :P