I had a similar issue - see post N800 & Netgear Router . Resolved by changing N800.

You mentioned in your troubleshooting post that you have two wireless networks, one on channel 1 the other on channel 11 - have you considered temporarily simplifying your network in order to try and narrow down the problem? Perhaps switch off one of the APs, and work "upwords" as it were. Your iPaq is connecting over 11b while the N800 will connect over 11g, in which case I wouldn't consider the iPaq a suitable "control" - do you have any other devices connecting over 11g? Your Squeezeboxen perhaps?

Is the firmware up to date on your D-Link APs? I myself have no problems connectig the N800 to a Linksys WRT54GS (running Sveasoft after market firmware). The DWL-2100AP is a 108Mbps device and may not be fully compatible with some 11g devices - check if you can disable the enhanced performance and see if it makes any difference.

Ultimately though it does sound like a routing issue. Can you pull up the admin web page for the associated AP on the N800? If you can, any problems beyond this point would appear to be in the AP/router rather than the N800.

If all else fails, do as Rab suggests and seek a replacement. If that doesn't resolve the issue, the problem lies with one or both of your D-Link devices.

By the way, I assume the access points have a built in DNS server (DNS Masq?) If so, why aren't you using the AP for DNS and leaving the AP to resolve DNS against your ISPs server - this usually works a little quicker, and may (or may not) help you. I found this post on the web which goes OT pretty quickly but does suggest there may be some odd DNS issues with some D-Link routers - try power cycling the D-Link devices to see if that helps.

I had a similar problem; it appeared to connect to the router but would go no further, even with static IP configuration. Took it back to the shop and they could not get it to work with their own WLAN but during the time we were looking how to do a reset it started working. So I took it home and it still did not work!?!? Anyhow, I checked carefully my router settings and I had enabled MAC filtering - added the MAC address of the N800 to the list and it worked fine.