It's definitely not good fir the mic, but it's only really harmful if the mic is constantly kept in a smokey environment.

There's a humongous risk of screwing your mics up with cigarette smoke. The smoke contains, among other things, tar, and the tar sticks to the membrane and kills the highs. However, that doesn't mean you cant record tobacco or in a smokey joint, it just means you have to be careful. All you have to care for is using some kind of protection (a rubber's not a good idea here ;-)), a regular foam mic-muff'll do just fine, and make sure noone really blows the smoke right into the mic.

Do it once or just a few times, and chances are you will now really notice much difference, depending on just how traind in listening you are. You will, however, probably notice it'll get harder and harder to mix the material as you want to though. But don't let that fool you, over time, all the small and tiny amounts of crap and filth that'll build up on the membrane will eventually have a tremendous impact on it, making it downright bad. Of course it's next to impossible to completely prevent deterioration and build-ups in the mike, but by being aware of what's harmful for it and protecting it from that as far as possible, you can maintain full audio quality for a very very long time :-)

Some time ago I did a video with a friend where we recorded a guy with a black-dark background where only the cigarette had some light, and wanted to record the sound of the cigar. I utilized a StudioProjects B1 condenser mic and, I mean, it was CLOSE, like, when we heard it, you could really listen to every tiny piece of tobacco being burnt. The cigar was no more than 5 centimeters from the capsule. Naturally, I got really scared haha, felt like it was going to get ruined (but it was one of my first recordings so I did not thought about the danger of my mic getting ruined). I did used the windscreen (foam) it came with.

Nothing wrong happened. Though, like sumdimsum said, I don't think it must be any good for it, but mine did not suffered at all :).

It is not just the tar that you need to worry about, there are other particulates that clog the grill and membrane. The only solutions are a pop filter and regular maintenance.