It's hard to have general guidelines for chattred stuff wrt what for instance rootkits try to overwrite or use (gcc), so in general I'd say stuff you don't want changed in /etc and sysbin dirs.
I use chattr to guard against (incidental) overwriting in case of root (wetware) or unwanted "guests" (incl. linuxconf) when a box has it's configs, libs and (system|chrooted) binaries fixed.
For configs I use it with a script that handles these permission bits + RCS, so I won't end up ever w/o a working config and track changes easily. With a well-configured Aide config (or Samhain, Tripwire) to track changes and a few aliases and scripts in place, handling chattred binaries and configs ain't that hard and shouldn't interfere much with "normal operation".
Btw, what do you mean by "Any word on that."? When 20-lq2 is out, or something about Grsecurity?