Raspberry Pi onboard temperature sensors
The Raspberry Pi does have an I2C bus on board, but it transpires that this is for connecting sensors to the GPIO headers. The Broadcom SoC has its own sensors, which can be read using the utilities in
# /opt/vc/bin/vcgencmd commands commands="vcos, ap_output_control, ap_output_post_processing, pm_set_policy, pm_get_status, pm_show_stats, pm_start_logging, pm_stop_logging, version, commands, set_vll_dir, led_control, set_backlight, set_logging, get_lcd_info, set_bus_arbiter_mode, cache_flush, otp_dump, codec_enabled, measure_clock, measure_volts, measure_temp, hdmi_ntsc_freqs, render_bar, disk_notify, inuse_notify, sus_suspend, sus_status, sus_is_enabled, sus_stop_test_thread, egl_platform_switch, vctest_memmap, vctest_start, vctest_stop, vctest_set, vctest_get"
measure_temp is what we're after:
#/opt/vc/bin/vcgencmd measure_temp temp=45.5'C
Anything less than 80ºC is OK. I run my RPi in a case without much ventillation and it never gets anywhere near this value.