Do 2 way devices such as the 2876DB ICON Dimmer only respond to status requests or do they periodically send out their status?
They only transmit to report a change in status and when the status is requested. Otherwise they remain quiet.
I think I remember seeing somewhere that the powerline can get swamped if there are too many transmitters.
What I think you are thinking of is the loading of the powerline. Each transmitter tends to reduce the overall signal strength (especially on that circuit). As a result some vendors set a maximum number of transmitters they reccomend you put on each circuit.
It would take about 30 of these to do my entire house and I do not want slow the powerline interface down much more than it already is.
It depends on a number of factors like how many of these switches will be on each circuit and how strong your signal is to begin with. The nice thing about INSTEON switches is that the INSTEON signal is much faster and it tends to get stronger with each switch you add so you can gradually migrate to the new technology while still taking advantage of your existing X10 devices.