These are the steps to enable I²C on Raspbian after downloading the new Raspbian “wheezy” image.

Add i2c-dev and i2c-bcm2708 to /etc/modules.

pi@raspberrypi ~ $ cat /etc/modules
# /etc/modules: kernel modules to load at boot time.
#
# This file contains the names of kernel modules that should be loaded
# at boot time, one per line. Lines beginning with "#" are ignored.
# Parameters can be specified after the module name.

# snd-bcm2835
i2c-dev
i2c-bcm2708
pi@raspberrypi ~ $ 

Install i2c-tools

pi@raspberrypi ~ $ sudo apt-get install i2c-tools
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Suggested packages:
  libi2c-dev python-smbus
The following NEW packages will be installed:
  i2c-tools
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 0 B/58.9 kB of archives.
After this operation, 223 kB of additional disk space will be used.
Selecting previously unselected package i2c-tools.
(Reading database ... 56011 files and directories currently installed.)
Unpacking i2c-tools (from .../i2c-tools_3.1.0-1_armhf.deb) ...
Processing triggers for man-db ...
Setting up i2c-tools (3.1.0-1) ...
pi@raspberrypi ~ $

Add pi to the group i2c if you want to access i2c without sudo.

pi@raspberrypi ~ $ sudo usermod -a -G i2c pi

Use i2cdetect to see if there are devices connected on the i²C-bus.

pi@raspberrypi ~ $ i2cdetect 0
WARNING! This program can confuse your I2C bus, cause data loss and worse!
I will probe file /dev/i2c-0.
I will probe address range 0x03-0x77.
Continue? [Y/n] y
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:          -- -- -- -- -- -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- 48 -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
70: -- -- -- -- -- -- -- --                         
pi@raspberrypi ~ $