Update... [pardon the lenghty details]
I took my iPad into the Apple Store in San Francisco yesterday. They ran some diagnostics on it but couldn't get any real info since I had tried a full restore the day before and no new data had accumulated to analyze. In the store, it definitely was having issues with a reliable signal strength. They didn't seem to know much about this issue, or if it was even an issue, despite me showing them this message board. Since it was within 14 days (mine was an online pre-order) they swapped it for an identical unit. It seemed a little more reliable in the store, but not incredibly better (flipping between 2-3 rainbows instead of dropping to just 1). So...I took it home, restored my backup, and used it around the house a bit. It was better, but still not as solid as my original iPad.
While at the Apple Store, I also purchased 2 AirPort Extreme base stations, just to see if that would help me at all. Given that wifi was better, but still not great, I swapped out my old WAPs with the AirPort Extremes. Previously, my network looked like this: Cable Internet -> Motorola SB6120 -> Sonicwall TZ210 firewall/router. I then had three access points plugged into the Sonicwall (24 port gigE switch in between). 1) DLINK DIR-825 which is in router mode, runs its own SSID, and issues DHCP IP addresses. This is my guest network and it routes guests straight out to the Internet 2) DLINK DIR-655 router running in bridge mode, runs its own SSID, but IP addresses are served from the Sonicwall. This SSID is my private wifi network; 3) Apple AirPort Express which is in router mode, runs its own SSID, and issues DHCP IP addresses. This is part of my private network, but it gives me coverage in a corner of the house the DIR-655 didn't reach (old house, lots of lathe and plaster walls full of wire mesh).
DIR-825 has latest firmware. DIR-655 has latest firmware that allows it to be a bridge - the latest and greatest removed the bridge mode, so I didn't completely upgrade it. Both WAPs run WPA2 Personal with TKIP/AES. This setup has existed unchanged since August 2007 and along with hardwired connections, I have something like 65 devices on the network (Sonos, Tivos, game consoles, lots of Apple devices, Windows machines, Rokus, etc.). I'd say about 2/3 are wifi devices and 1/3 are hard-wired. The network has been bulletproof for everything - except my first new iPad. The replacement new iPad seemed better, but still not perfect, as mentioned above.
So...I left the DIR-825 alone as that is primarily used by guests (sitters, relatives, etc.). I replaced the DIR-655 and the AirPort Express each with an AirPort Extreme base station (upgraded to 7.6.1 firmware). I configured them in network roaming mode (they both uplink via GigE, but they are in bridge mode (DHCP coming from Sonicwall)), run the exact same SSID and WPA2 password. The roaming network allows you to roam between access points unbeknownst to the end user, since you aren't changing SSIDs, getting new IPs, etc. The swap was pretty painless, and all my old devices immediately reconnected (I used the SSID and password from the DIR-655 which almost every wifi device I have has configured in it). I then played with the replacement new iPad and it works flawlessly everywhere. I can sometimes death-grip it from 3 to 2 rainbows of strength, but I can't get it to drop. And it is now blazing fast with speedtest.net app showing ~45mbps down and ~11mbps up - just as fast as my wifi iMac and MacBook Pro and only slightly lower than my gigE Win7 desktop.
I should also mention - neither my first new iPad nor my replacement new iPad (or my iMac for that matter) would see any of the 5GHz SSID broadcasts from the DLINK equipment. My MacBook Pro did though, and that's what I typically used. With the AirPort Extremes, my iMac and replacement new iPad both see full signal 5GHz broadcasts - so that's what I'm connected to and that's what I used when doing the performance testing with speedtest.net.
So...what made the biggest difference? I don't know for sure. It seems this iPad is better than the first, but in combination with the new WAPs, it seems totally fine.
I wish I had better advice as I know a lot of folks here have AirPort Extremes and are still having issues, so there is something combinatorial going on. But in my particular case, I think I'm good to go now (at least in my house).
Hope this helps someone either figure out what is up or at least work around the issue and get satisfactory reception from their device.