If the Linux kernel community had to exist somewhere physically, it would call the Linux Kernel Mailing List home. The Linux Kernel Mailing List (or, as the regulars abbreviate it, just lkml) is the location of the majority of the announcements, discussions, debates, and flame wars over the kernel. New features are discussed and most code is posted to the list before any action is taken. The list sees upward of 300 messages a day, so it is not for the faint of heart. Subscribing (or at least reading a digest or the archives) is recommended for anyone interested in serious kernel development. You can learn a lot simply by watching the wizards at work.
subscribe linux-kernel <firstname.lastname@example.org>
Numerous websites and other mailing lists pertain to the kernel specifically and Linux in general. An excellent resource for beginning kernel hackers is http://www.kernelnewbies.org/a website that, of all things, caters to those cutting their teeth on the kernel. Two other excellent sources of kernel information include http://www.lwn.net/, Linux Weekly News, which has a great kernel news section, and http://wwww.kerneltraffic.org/, Kernel Traffic, which includes a summary of the previous week's lkml emails with insightful commentary.