I have an application that transmits data once per minute, and I chart the RSSI. I've seen it occasionally as low as -90dBm, maybe even a couple less. Interesting that during the night when the house is quiet (electrically as well as otherwise), signal strength is stronger and very consistent. When I'm in my office where the one node is, with two or three PCs running, and lots of fluorescent lights with electronic ballasts, things bounce around much more.
The other interesting thing (I'm using S2 XBees) is adding a third node physically somewhere between the other two. This will sometimes cause an increase in signal strength because the traffic will sometimes switch to be routed through the intermediate node. So that's one way that the mesh networking can be seen doing its thing 
The RSSI for the intermediate and the furthest nodes will then be nearly identical -- RSSI only reports the last hop.
