You can tell a Vim server to reload the vimrc file like so:
$ vim --servername MARTIN --remote-send '<Esc>:source $MYVIMRC<CR>'
And you can get a list of all servers with:
$ vim --serverlist
Which can be combined with a
$ for s in $(vim --serverlist); do vim --servername "$s" --remote-send '<Esc>:source $MYVIMRC<CR>'; done
Since that's a lot of typing, you could put it in a shell script in
~/bin/reload-vimrc; you can of course also call it from within Vim:
If you're running Linux, and want to get really crazy, you could use
inotifywatch from inotify-tools to monitor your vimrc file(s) for changes, and execute this automatically on every change (untested, example adapted from manpage)
$ inotifywatch -v -e modify -t 60 -r ~/.vimrc ~/.vim
I wouldn't recommend this, though ... If you accidentally save a file with a
syntax error you could bork all your vim session...