I'm not sure if IPv6 in Ubuntu 10.04 Lucid Lynx is still causing issues with the Internet connection being slow and so on, but in case it does here's how to disable it.
If your browsers work OK, you do not need to disable IPv6. Also, there is nothing wrong with either Ubuntu (Karmic) or IPv6, so if you are experiencing any issues, they are because of your ISP / router / modem.
I found a really quick fix today for disabling ipv6 completely on Ubuntu 7.10 (not yet tested on previous versions). This might be of interest to some of you that have had networking problems, as I’ve heard disabling ipv6 at least within the browser has been a help here.