In my experience, the nature of the relationship and the nature of the communication are both important for knowing when/how to use the plain form and to knowing what the use of plain form signals.
In written workplace communication, I never see plain form (I work at a university).
In written personal communications (things like Facebook or IM), I rarely see ~ます forms. Only in the most formal invitations do these crop up.
In verbal workplace communication, I've seen quite large variance among language users. Some users almost invariably use the polite forms; other users use plain form except when giving a formal communication (e.g., say starting a meeting or being the person who is handling the meeting point). One parameter that affects this is the role/position one has in the organization and meeting.
Consequently, I am much more careful to use polite forms in 運営委員会 than when I am talking to my own students. Though note, that the use of polite / plain is almost universally mixed in the communication I encounter. Some professors insist on polite forms from their students and use 僕 to refer themselves; others don't care whether students give them polite forms or not.
My advisor when I came to Japan generally used the plain form when talking to me, but the first sentence was often in a polite form. I generally but not universally use the plain form when talking to my students. Some professors (who sound refined when they do it well) stick to the polite form for almost all communications in person.
In verbal communication among friends, I would say that to close friends, I rarely use ます forms (です remains however in some contexts). To people introduced to me by friends, I will generally do the introduction in polite forms and quickly transition to plain forms for most communication if that's the pattern they are talking with my friends in. To people I meet through other means, I will start with ます constructions and stick with them until I feel comfortable around the person.
I might suggest the following equation where P(x) is the probability I use a ます construction:
P(x) = A x [Their age - my age ] - B x [length of time I've known them] + C x [business relationship > government office > acquaintance [~0] > friend of friends [< 0] > direct friend relationship [<< 0] ] - E x [depth into the conversation/message] + F x [written > 0 | spoken < 0]
Where A, B, C, D, E, and F are positive constants that vary somewhat by the individual.
We could call it the "virial politeness law" (Cf. http://facstaff.cbu.edu/rprice/lectures/realgas.html)