With the move from OS X 10.5 to 10.6 Snow Leopard several legacy applications will no longer work as there will be no support for the Power PC code that they are based on.

I have CAD files going back to the mid 90's sitting on my hard drive, thousands that were created in Vellum 2D / Graphite and thankfully way less in Vellum Solids, Cobalt and early versions of Concepts Unlimited. The Concepts files are I think safe in that they will open in Shark, but the Vellum, Vellum Solids and Cobalt files are not. Here is what I am doing.

Vellum 2D + Graphite 2D files
Convert to DXF / DWG

Vellum Solids + Cobalt files
Convert to ASICS SAT

I am using an old copy of Cobalt V6 and Graphite V7 that still runs to do the conversion, but it is unlikely that either of these application will even launch once I have upgraded to 10.6 Snow Leopard.

EDIT
Correction
Rosetta which is required to open PPC based applications is an optional part of the 10.6 install, so you will be able to open these old apps, but only for as long as Rosetta is around. In other words, it is likely to be gone by the time we get to 10.7 so if you want the data you still have some time to do the transfer.