I have no plans to have it out on rides (at least not on the roller coasters). It will be safely ensconced in its case.
I know what you mean. I've had my eye on the D90 for a couple of years now, right up until the D3100 came along. But here's what edged me toward the latter:
1) While the D90 has scads of great features over the D3100, the D3100 has a bigger, better sensor. As nice as the D90 is, it's almost 3 years old now, and DSLR technology has come a long way in that time. This is one key area where it shows.
2) I have *SOME* SLR knowledge (I worked in a camera store for about 6 months -- albeit 9 years ago), but very little practical experience, and my wife has none. So we both need a camera that is going to be tolerant of our inexperience. The guide mode on the D3100 seems ideal for learning the shooting basics.
3) Similarly, while I'm comfortable with the heft of the D90, my wife would need something lighter. It's a catch 22 for me because while I'd actually favor a heavier camera for shooting (holding both in the store, I favored the D90), I'd certainly prefer lugging the lighter D3100 around the other 95% of the time. Thus, I'm erring on the side of portability.
I have no doubt I will want to upgrade the body within two or three years. But by that time, I'll actually have enough skill under my belt to warrant something like the D7000 (or its successor).
Making my way through "Understanding Exposure" as we speak.