Wall dimmer switch is warm to the touch after leaving the light on - cause for concern?
If this was a normal switch, I would say, no this is not normal.  Since the switch does contain a dimmer, it is perfectly normal.  Dimmers are electronic devices that do heat up. In fact, because of the heat they create, dimmers are rated for a maximum wattage.  Special high wattage dimmers are available, and they even have integrated heat sinks to help dissipate the heat they generate.

That said, you need to calculate the total wattage of the bulbs you are controlling with the switch.  If you have 6 bulbs, the max wattage you can use is probably 40W.  A typical residential dimmer will have a maximum of 600W (yours will be labeled, probably on the face where its easy to see).  Using 12 40W bulbs you have 480W, but using 60W bulbs you would be over-budget at 720W.  This is important, because when you are over 600W, the dimmer will still work, but it could possibly get hot enough to damage itself or the wiring connections in the same box.