Same problem here. We have 3 TiVos connected to our network. Two of the TiVos have the TiVo branded network access point, the 3rd TiVo has a LinkSys access point. There was no problem when I was using an Airport Extreme Base Station (AEBS). I switched to a TimeCapsule last week, which I set up as WEP security right from the start. Network Preferences said WPA personal. The two TiVos with the TiVo branded access points could connect, the TiVo with the LinkSys access point could not. Now my TimeCapsule is malfunctioning so I switched back to the AEBS. It is configured with WEP, and the Network preferences still say WPA personal, but now all 3 TiVos can connect to the network, even the LinkSys access point.
Moral of the story, if your TiVo can't connect to your TimeCapsul, you have to upgrade to a WPA capable access point.