My brother in Australia had the same problem with his D-Link router during 10.5.1...
It appears to be an endemic fault in it's compatibility with Leopard.
I was down there over Christmas, and I was trying EVERYTHING - the only thing that I could do to make the internet connection work was to take out WEP or WPA encryption, and then voila! NOT something to recommend, though - so we put it back and had to suffer with a cable, leaving us scratching our heads (not good form to have an ethernet cable sticking out of the back of a Macbook, though...). Now, he's ordered an Airport Time Machine box - and I'm sure that's guaranteed to work.
Funny thing is that when I recently upgraded to Leopard (two weeks ago), suddenly my internet connection became erratic, slow and just ridiculous. I could still talk to my Netgear router, but I'd have to restart the router and the computer's Airport connection every once in a while to reset the internet. So I KNEW it was a 10.5 problem.
I went down to the local Apple Store (after reading the plights of many hundreds of people on Apple's own discussion boards who have the same issue - and found Apple ignoring them), and asked for a little help. The guy pleaded ignorance (great strategy - it's not a problem if you don't know about it...), but suggested to search the Netgear website for any firmware updates.
This I did, and have now found my router to be VERY reliable....
So - may I suggest for you to go to D-Link's website and download and update your router's firmware to it's latest version? It might help...
Let me know how you get on!
🙂