My case was exactly the same, see my timeline below. I went to USCIS today (2-15-06) to get BIO done, I had received an invitation 3 weeks ago. The strange thing is that they did Code 2, not 3, which I have never done. They were not able to answer my question why. I have been approved since 12-2-05.
I think you can't just schedule yourself for a BIO, but I may be wrong.
Myself - Derivative:
ND: July 20 2002
FP: Nov 2004
Medical: Received by USCIS 09-29-05
APPROVED 12-2-05!!!
BIO: 2/15/06, Code 2
GC: Waiting
My Wife - Principal:
ND: July 20 2002
FP: Nov 2004
Medical: Received by USCIS 09-29-05
BIO: Code 3 Done on 09-27-05
LUD: 10-03-05
APPROVED 1-10-06!!!
GC: in hand 1/14/05