Ok sounds like it showed up once. Something changed since then and not it wont. Need to see if you can get back on the router and if you see it listed under DHCP Client, I would suggest you Reserve the IP address for it.
I would do this, do a factory reset of the router, set G mode only with out any security and see if you can get the DS to connect and reserve the IP address. Turn off all other devices as much as you can. I think you might have to do some tweeking of the WiFi, i.e. transmission rate and channel to see if it will connect.
I would also verify that the DS unit works elsewhere, friends or neighbors network? This will let you know that the DS unit is good. Might see if someone else has another DS that you can try with this router too if you can.
I feel your router is working well, it's the possible configuration of the DS and or the WiFi on the router is getting hosed up.
I think you should try to take the time and set up all of your devices to Reserved static IP addresses on the router. This will help in troubleshooting, management and setting up other options on the router. Also each device will get an IP address that doesn't change. Sometimes using DHCP addresses can break a option setup on the router, i.e. rules, should the address change by accident or end of lease. I have over 15 devices in my house. There all set up with there own IPs.
Just a suggestion.