> Extract it into /usr/local/src/linux instead.

make a dir under $HOME for your kernel source. that way you don't have to be
root to work on it. (or muck with permissions on the /usr/local/src tree).
I understand this is what Linus does.

/usr is supposed to be immutable (at least as I understand the filesystem
standard). you should be able to mount it read-only, under ideal
circumstances.
/usr/src is a legacy from when user's home dirs were under /usr. really, one
should consider making a /home/src tree, symlinked from /usr/src, if one
uses /usr/src on a regular basis (such as those of us building RPMs).
(not that I've actually tried this; it's just an idea I'm kicking out).

Carl Soderstrom.
-- 
Network Engineer
Real-Time Enterprises
(952) 943-8700