I think it's more likely that your TERM variable isn't set correctly, rather than there being a defect in nano. Basically, if the TERM variable is not right for your terminal program, then there will be screen drawing errors (i.e., if your terminal is emulating VT100, but your TERM is set to xterm, it'll use codes that will not redraw the screen correctly, especially for an application like nano)
What client are you using to access the box, at least in terms of using nano? Can you do something like
echo $TERM (before invoking nano) and tell us what the result is?
The terminal config files are in /usr/share/terminfo somewhere. On my CentOS 6 box, I see that the x/xterm-256color file is part of the "ncurses-base-5.7-3.20090208.el6.i686" package. Do you have that package installed? Run
rpm -qa |grep ncurses and it should show up (I think nano only depends on the ncurses-libs package, so ncurses-base may not be installed). If not, run "yum install ncurses-base".