BrisCAD is probably the only true alternative to AutoCAD that also supports the DWG file format. Recently Bricsys, the creators of BrisCAD, released the Linux version of Bricscad V10.
Bricscad V10 for Linux supports LISP, DCL and Diesel APIs, and about 500+ third party application developers are already porting their apps for the Linux version.
At the time of writing, Bricscad V10 works on Fedora 12 or higher, OpenSuse 11.1 or higher and Ubuntu 9.10 or higher. Note that Bricscad V10 is not free. However, you can download the 30 days trial version for free.