Not that I use Google Desktop Search myself, but the General CADD people seem to be in the forefront in getting it to support CAD formats. Writes Jim Faliveno:
We added three more file formats to this version of the Free Google CAD Desktop Search Indexer: DGN (MicroStation), GCD (Generic CADD), and VCD (Visual CADD).
That's on top of DWG (AutoCAD), DXF, and and GXD (General CADD). Note that the utility searches for text strings stored in CAD drawings. Click to download the General CADD Desktop Search Plug-In.