read only system file systems for jail

[Chad Leigh -- Shire.Net LLC]

Hi All

I am playing around on 5.2-CURRENT and am setting up a system to run 
various programs inside of jails.  Including allowing the users to ssh 
in etc.

Is there a fundamental problem of having the following all be read-only 
file systems, with the noted exceptions?

/bin
/sbin
/libexec
/lib
/usr
/var

note:  /usr/local would not be readonly and /var/tmp would not be 
readonly

It seems to work in my test jails but I was wondering about hidden 
problems or non obvious problems.

note that users are not allowed root privilege and hence are not 
installing stuff into any of these hierarchies and no /usr/ports

Thanks
Chad