'resolv.conf's should push and pop in an orderly fashion, so if you are saying that you make a change and then it permanently disappears, then something odd is happening. On the other hand, if, when some application such as NetworkManager or Wicd takes control of networking, you temporarily see something unconnected to what you originally set, then that is expected behaviour. But along with this expected behaviour, you should see some note along the lines of "### altered by network manager...do not edit" and probably a date/time that this last happened.
While NM/wicd are in control, any settings available from DHCP/mDNS might turn up in the file, although you can probably reconfigure them to only take selected parts of the information.
If, of course, you did edit the file while it had the "do not edit" comment present, you should expect things to go wrong, even if you don't know quite how soon this should happen.