It seems quite a few people are upset about Firelands not being in patch 4.1. This is being seen as the insult added to the injury of Cataclysm, which is rather light on content.1.
Of course the object of criticism here may be entirely fictional. The fact that Firelands is going to be in 4.2 rather than 4.1 probably has absolutely no impact on how soon we will see Firelands live on the servers.
Suppose instead that Blizzard had announced that because 4.1 was going to take longer than they anticipated, they were going to do a patch 4.0.7 that had some of the content they originally planned for patch 4.1, such as the troll dungeons. The PTRs would be changed to say they were testing for 4.0.7, the patch notes would tell us what to expect in 4.0.7 and so on.
This would be exactly the same thing as what is going on now. Version numbers are totally arbitrary. Even if there are rigid rules that govern how they work they are still totally arbitrary. If people are upset that it isn't coming as soon as they had hoped then that's one thing, but version numbers shouldn't be part of that equation.
Of course from that perspective I think I can agree that no Firelands in 4.1 is a bad move. It's a bad public relations move, though, not a bad game design move. They should release Firelands when they release all of their content: when it's ready.
1, Cataclysm wasn't actually content light. It had tons of content, but a good amount of the content was aimed at characters under level 60. This may have been a mistake - it might have been the game design equivalent of teaching an old dog new tricks. Still, any criticism should be about poorly targeting their efforts, not phoning it in.