If anything, DirectX will be less and less of a restriction on PC development the more titles are multiplatform and/or utilize major gaming middleware. PS3, WiiU, Android, and iOS are all OpenGL-based platforms; all new multi-platform middleware systems support OpenGL in order to work with these platforms. On a purely graphical level, most games (especially multiplatform ones) could support OSX and Linux pretty easily going forward. (It's really other elements of the system -- and dealing with bad drivers -- that make porting time-consuming at this stage.)
Do not underestimate this.
OpenGL is great on platforms with little hardware variance, like consoles and mobile. Even in OSX, hardware diversity if very low when compared to PCs.
On PC hardware, if you want to go beyond baseline shader model 3.0, it's a nightmare. DirectX 10 and on really streamlined the whole feature set stuff: DX10/10.1/11/11.1 cards are guaranteed to support well defined groups of features, whereas with OpenGL you need to deal with extensions and having to check features one by one.
Not to mention that even with DirectX, new high-profile games often have to deal with driver issues, both on NVidia and AMD cards, when they start pushing GPUs in "interesting" ways. In OSX it was a hundred times worse: the arrival of Steam and SC2 improved things somewhat, but it's
still impossible to make Mac games that use shader model 4.0 or rely heavily on render buffers with formats other than R8G8B8A8, for example. The situation in Linux, considering the drivers aren't fully open source, is likely to be similar.
Source Engine will have no problem in Linux because their rendering techniques are mostly outdated, from a technological point of view. Many indie games, which usually tend to be low on the GPU trickery, are also good. But stuff like Skyrim, as example, is unlikely.