I stand corrected. I thought I remember seeing on some site they both had the same panel but I guess not.
It is very similar though, as mrk says above the Dell is S-IPS and the NEC is AS-IPS, which is just what LG Philips call their latest generation of IPS panels. So not exactly 'completely different' then, more like the next generation of the same panel technology. According to tftcentral.co.uk, AS-IPS allows higher brightness and improved colour reproducibility, so I would guess the response time difference (8ms GTG for the Dell vs 6 ms GTG for the NEC) is just down to either measurement technique or NEC using more agressive overdrive.