I have a Dell 8600 with centrino as well. I think I have the same Intel Pro Wireless LAN 2100 3B mini pci card in my laptop.
When I first set up wireless networking from the asus router, I couldnt keep any connections either.
It turned out that windows xp wireless config and the Intel wireless config program that dell had installed were competing over which one should have control of the card.
Once I turned off the Intel program, most of my problems went away.
Since I set up the asus with "broadcast ssid" turned off, I got other problems. It turns out that the built-in windows xp wireless config has a tendency to drop connections to access points with "broadcast ssid" turned off, in favour og access points that broadcasts ssid. According to MS; "it's a feature" . Unfortunately, I live in an apartment building, and one of my neighbours has a wireless accesspoint with broadcast ssid turned on...
I sorta overcame that by turning up the signal strength from my router to 50mW. I don't know why it helps, but I don't have any lost connections anymore.
Don't know it any of this helps you, but it might give you some ideas to try.
I've been running Olegs 1.6.5.3-3 and 1.7.5.6-2 firmwares, and the signal strength has been set using nvram commands.
best of luck
Royan
Edit: Link.