The Doomworld Forums have an interesting follow-up to the fact that DOOM 3 made use of a "zfail" method of rendering shadows that is patented by Creative Labs (thanks Ant). Recalling this, they asked id's John Carmack how this might impact plans to release the DOOM 3 source code under the GPL at a future date. Sure enough, this situation could potentially result in patent infringement:

When we release the code (no date set), anyone that uses it would potentially be infringing. There are workarounds at a modest performance cost.