My personal theory on why they haven't updated it yet is two-fold:
1) they may not be able to cram the extra antenna(s) in (802.11n uses MIMO (multiple in multiple out)... i.e. multiple antennas on each station). I'm not clear on what the minimum number of antennas is.. it doesn't seem to be standard.
2) they want to push as many people to Apple TV as possible.
If they don't have an 'n' Express out by the end of the summer I'll probably just cave and buy the 'g'