I have an n800 and the finger detection for input fields is not perfect, however I noticed a few observations which may be of use to you.

1) When tapping with the stylus, no matter how much pressure is applied it never brought up the finger keyboard.

2) When using my finger, if I used the front of the finger (near the fingernail) it usually wouldn't bring up the finger keyboard, if I used the bottom of my finger (as if I was trying to stamp an inked fingerprint) it would bring up the finger keyboard almost all the time.

3) If I pressed the input field with two fingers at the same time it would always bring up the finger keyboard without fail, and this is even if I used the tip of the finger. I even tried it with two styli, as long as the two presses were more than 3mm apart it would bring up the finger keyboard.

I hope this information is somewhat helpful.