Hi,
I am trying to configure a RB260GS switch to untag tagged traffic on one port and all of the untagged traffic to all other ports. The issue is that I cannot understand how to accomplish that.

The setup is MT-Router -> EdgeSwitch -> Dumb switch -> RB260GS

On the EdgeSwitch I am both untagging vlan 10 (access port) and tagging vlan 40 to the "Dumb switch"
Port 1 on RB260GS is connected to the "dumb switch" and I would want to be able to untag vlan 40 to port 5 on the RB260GS. All the other traffic should be untouched and go to ports 2, 3 and 4. Is this possible?

Well I did get it to work... this is how I did it if it would be helpful to others
Vlan 40 is the tagged vlan that I wanted to untag on ports 4 and 5. The RB260GS treats the untagged traffic coming from the trunk port as vlan 1. Vlan 20 is left there because I was testing and forgot to delete it

Take a look at this. Not exactly what you are doing, but should help you out.
http://wiki.mikrotik.com/wiki/SwOS/Router-On-A-Stick