A sorry saga indeed! ADSL up and running with D-Link 300G+ modem, all OK.
When I first attempted to introduce a Linksys WRT54G wireless router, big problems! Neither the ADSL provider nor the Linksys helplines could explain how to configure the router properly, and the Linksys guy was particularly unhelpful. A Saturday afternoon's worth of head-scratching eventually produced a result, and I had my little WLAN set up. So far, so good. My very remote B&W was finally on-line, and I was thrilled to be able to go to ckua.com and listen to my favorite radio station. I was surprised to find that the 20Kbps stream wasn't reliable. I know that I'm pushing the limits of signal range down here, but 20K isn't a whole lot. But, I thought "what the heck!" I'll move the router a little closer, which I did with a certain amount of wires all over the place, just temporary of course. I didn't change anything else except the physical disposition of the hardware, and now I can't connect via the WLAN or even with the i-Book cabled to the router. I've done factory resets on the modem and the router, and then configured them as they had been when they worked together, I have no problem connecting via the ADSL modem cabled directly to the i-Book, nor indeed directly to the modem to configure it. The wireless function of the router works just fine and can access it's set-up just fine too, but for some strange reason the router isn't talking to the modem. I can't even connect to the modem's set-up pages via the router (and not even directly with Safari, but that's another issue, IE works OK).
I've changed the microfilter. checked the cables, the network prefs, done factory resets and reconfigured, and I'm stumped!
The ADSL modem is configured as per the ISP's info, and works out of the box with a cabled ethernet connection.
The routers config is:

IP address: my fixed IP assigned by the ISP

subnet mask: 255.255.255,0

gateway: the gateway IP address of the modem (the fixed IP + 1 in fact)

DNS: as per ISP's info

This did work, and now it doesn't. Have I done something very stupid and very obvious?

(Signed)

Obviously Stupid in Bedfordshire.