When players of a multi-platform game compete online, are they segregated by platform? For example, do PC players of Left 4 Dead ever meet Xbox players?
In general, the online components of games are segregated by platform. However, it's fairly common to see PC and Mac users on the same game servers (TF2, for instance).
Mac/PC aside, there are very, very few games that support cross-platform play between consoles and PC. Shadowrun was one such game which supported Xbox 360 and PC cross-platform play. I also believe that Portal 2 featured PS3/PC cross platform co-op.
Giant Bomb has a list which is probably incomplete, but likely the best source on this subject. Do note that they consider PC/Mac to be cross platform enough for inclusion on their list, so things like TF2 and L4D are on here, but may not fit the criteria of your question.
Yes, most games separate players from different versions. For consoles this has competative reasons. XBox Live and PS Network do not share user information.
But the separation between Xbox Live and PC is done mostly because of the following reasons:
As both arguments apply similarly to PC/Mac they often share the same user base.
I had a talk with a yearly AAA series's chief designer about this issue.
I was disappointed to hear that the main reason for multiplayer for the same game being split by platform is due to licensing & market reasons. Sony & MS don't want their established player-bases mixing for a variety of reasons (eg. perceived advantages in game leading gamers to jump ship; attraction of a competitor's value-adds, exclusivity for releases etc).
There's also the fact that shared servers mean that Sony or MS can't control the online ecosystem, so there's less chance of revenue through online ads & statistical information that can be passed to marketing.
... Which is all a pity because TCP/IP is standard across all systems so there's no technical reason why the games can't communicate. It's all the same code at the protocol level.