I am having the exact same issues as most with Airport on both my iMac and Macbook since 10.5.2. I am in Japan and use NTT (AT&T for Japan basically).
I think the problem resides with Leopard vice anything else. I have a Dell laptop with XP and another Dell with Ubuntu that both run flawlessly as did the Macs before Leopard. If I have a wired connection everything is good, it is only with systems with Leopard that are giving me problems when wireless.
I have a D-Link DIR-625 router and have never had issues before now.
The only time I had any consistent luck was with 10.5, unfortunately I am going back to it full time now at least on my MacBook.
I have a DLink as well, though its the DGL-4300. I was having problems in 10.5.1 with my Powerbook. EVERY time I'd wake it from sleep it couldn't find my wireless network.
The short-term fix was to switch my SSID to 'Broadcast'. Not particularly the solution I care for, as I like a multi-layered defense (MAC address filtering, WPA-2, and disable SSID broadcast). Anyhow, maybe that will help someone.
BTW, I NEVER had that problem in Tiger. Bottom line - Leopard's airport drivers SUCK.