Once a paper of mine has definitively been accepted for publication, I typically list it on my CV and in citations as "forthcoming". Other people seem to use "in press" for the same category, and an author once asked me to change my reference to his paper to use "in press" rather than "forthcoming". Do you feel that there's a literal difference in what's conveyed by these two terms, or some difference of connotation? I suspect it simply has to do with conventions in different disciplines.
(Personally, I'm a little bit uncomfortable with "in press", since it suggests that the publisher is at least close to the process of printing the paper. Apart from the fact that that doesn't make sense for online-only journals, I feel funny referring to a paper that I know will appear in a journal issue dated a year and a half into the future as "in press". Actually being printed is a long way off.)
(On the other hand, I suppose that "forthcoming" might be confusing to someone who doesn't know its standard meaning in academia. In theory, someone might interpret it as implying no more than "This is something that I'm planning to write, when I get around to it, and this is the journal I expect to publish in." Obviously, that's not what most academics take "forthcoming" to mean, but any thoughtful person would understand that "in press" implies that you can count on the paper being published.)
EDIT: Obviously, given some of the answers to my question, the way that I thought that nearly all academics obviously interpret "forthcoming" was not correct (assuming that there's not too much weirdness in the sample I've drawn by asking the question here).