I seriously fail to understand this fascination about politeness in speech among some Japanese learners. I believe that if you are originally a polite person, then you would naturally speak politely in Japanese. If you are not, you will speak accordingly, in Japanese or your native language.
I wouldn't exactly call it fascination. A better word might be paranoia.
For myself, I often worry excessively about rubbing people the wrong way with what I say. It takes me forever to write formally in English, because I take so much time making sure there's nothing at all that could possibly offend someone.
As for others, I think it's part of how we are taught. A lot of emphasis, perhaps too much, is placed on being formal. You will not find a book or CD set that even mentions plain forms in the beginning. My previous grammar book has 40 lessons, and it doesn't mention plain forms until Lesson 12. The dialogues are in plain form for only two lessons, 34 and 35. Everything else is formal, formal, formal. That, and some other factors, put this fear into you that if you incorrectly speak in plain for even one sentence, the Gods of Formality will unleash their wrath on you and beat you up in a dark alley somewhere.
EvilCowSlayer, may I ask you to explain what formal and informal situations are to you?
In English, I pretty much speak informally all the time. It's difficult to think of situations where I don't. If I'm a guest at something, or if I'm talking to someone of significantly higher status, I tend to speak a bit more formally.
In Japanese, I try to imitate how others are speaking, but sometimes I see a bit of both, and I get confused. Currently, and I believe a lot of other people are using this too, the method I use is a "safe" one: informal with family and friends, formal everywhere else. This mostly stems from the fact that we're not sure; it's never really been clear cut to us. I don't like to use it, but it's a necessity, as we're never really told when to use each.