HackADay posted a link to an article (in Spanish - translated to English) with some thermal images, I'll summarise it:
There are three main sources of heat -
The input voltage regulator (bottom left).
This regulates the input down from Vcc to 5V. Assuming this is a linear regulator, power dissipation will equal
I is the current usage of the device.
If you supply the board with 6V, and draw 0.5A, the power dissipated by this chip will be (6-5)*0.5 = 0.5W.
SoC BCM2835 (middle). This contains the ARM11 CPU, GPU and RAM.
LAN951 (right). This is the USB/Ethernet controller.
The following tests were performed at an ambient temperature of 26-27°C with no draft.
Idle - no Ethernet:
Idle - Ethernet connected:
- Ethernet controller temp increases to 55.7°C with a maximum of 59.5°C.
CPU Stress Test:
- CPU temp increases to 55.7°C with a maximum of 57.9°C.
High Definition Video Playback:
- CPU/GPU temp increases to 56.1°C with a maximum of 58.6°C.
- Ethernet controller temp increases to 60.8°C with a maximum of 65.1°C.
From the above tests we can conclude that the hottest component is the Ethernet/USB controller, with a maximum of ~65°C (37°C above ambient). The CPU/GPU reaches ~59°C (31°C above ambient), whilst the voltage regulator reaches ~59°C (31°C above ambient).