After spending two hours fighting different WRT54GL firmware versions, WDS, Client Bridge, and other stuff, here's my solution for bridging the ethernet to WLAN on an Linksys WRT54GL with a Fritz!Box Fon WLAN 7140 WLAN basis station.

• Get dd-wrt for the WRT54GL. I used version 24-sp1.
• Do a full factory reset on the WRT.
• dd-wrt Setup: Basic Setup/Network Setup: configure an IP in the same network as your Fritz!box is in.
• Security/Firewall: SPI Firewall -> Disable
• Wireless/Basic Settings:
• Wireless Mode -> Repeater Bridge
• Wireless Network Mode -> G-Only
• SSID -> needs to match your Fritz!box SSID
• Network Configuration -> Bridged
• Wireless/Wireless Security:
• Security Mode: if your Fritz!box is configured for WPA or WPA2/Mixed, select WPA here (NOT WPA2! - it won't work)
• WPA Algo should be TKIP for a Fritz!box
• WPA Shared Key -> same as on your Fritz!box

You probably need to reboot the WRT after changing all this stuff.

Now go to Status/Wireless, you should see the Fritz!box in the Access Points & Clients list, with a Signal Quality > 0.
If it doesn't work yet, try the "Site Survey" and click "Join" next to your WLAN network.

There isn't anything special to configure on the Fritz!box. If it finds your WRT it should say "Repeater" next to it in the WLAN Monitor menu.

Probably it's better to use WDS instead of "Repeater Bridge", but I haven't got that working. If you do, let me know!