Here are my results:
HDE temp laser gun
Values are in Fahrenheit
Temp readings are of hottest areas on top and bottom of router. Usually right hand side, just about a 1/2 inch from the vent louvers on the smooth surface and about 2 inches from the right side edge of the router.
Room temp where router is located: 70-71F
Temp at IDLE: Top - 111F/ Bottom - 120F
Temp with only 2.4Ghz data flow: Top - 109-111F/ Bottom - 120-122F
Temp with only 5Ghz data flow: Top - 109-110F/ Bottom - 120-122F
Temp with both 2.4Ghz and 5Ghz data flow: Top - 108 - 111F/ Bottom - 122-124F
Temp with wired LAN, 2.4Ghz and 5Ghz data flow: Top - 110-114F/ Bottom - 122-123F
Inside vent louvers: 112-114F
Temp test with only 2.4Ghz data was streaming DVD movie on HDD from DIR-685 connected via wired to DIR-857 to a Mac Book Pro using OSX 10.6.8. Wireless and DHCP services disabled on DIR-685. Just working as a NAS device.
Temp test with only 5Ghz data was streaming DVD movie on HDD from DIR-685 connected via wired to DIR-857 to a Mac Book Pro using OSX 10.6.8.
Temp with both 2.4Ghz and 5Ghz data was streaming DVD movie on HDD from DIR-685 connected via wired to DIR-857 to a Mac Book Pro using OSX 10.6.8 via 5Ghz, while Boxee Box was streaming on 2.4Ghz DVD movie files from my file server that is wired Gb Lan connected to the DIR-857.
Temp with wired LAN, 2.4Ghz and 5Ghz data was streaming DVD movie on HDD from DIR-685 connected via wired to DIR-857 to a Mac Book Pro using OSX 10.6.8 via 5Ghz, while Boxee Box was streaming on 2.4Ghz DVD movie files from my file server that is wired Gb Lan connected to the DIR-857. Xbox 360 was streaming Netflix movie via hard wired LAN cable connected to the DIR-857.
Over all, never saw much Top temps get over 112-114F on the top of the router and the bottom always seemed to maintain nothing over 122-124F. Took me about an hour and a half to test as I graduated from a single streaming device to adding all 3 for just this test. Taking readings every few minutes to see if router temps would fluctuate greatly. They seem stable for the most part and didn't see any router problems at all. I'm wondering if there could be Mfrg issues or differences in some routers.