After patch 1.3, this is no longer needed. If you haven't yet, turn off V-Sync to reduce mouse lag!

Prior to patch 1.2, Skyrim had an odd issue with the vertical mouse movement being synced to the frame rate. After patch 1.2, you should now experience consistent mouse movement regardless of frame rate (now you may want to disable v-sync), but your vertical and horizontal movement may still be out of sync. If they are, you can try adjusting the fMouseHeadingYScale and fMouseHeadingXScale variables, which can be found in the [Controls] section in Skyrim.ini:

Windows 7:
C:/Users/USERNAME/Documents/My Games/Skyrim/Skyrim.ini

Windows XP:
C:/Documents and Settings/USERNAME/My Documents/My Games/Skyrim/Skyrim.ini
These variables and the [Controls] section may not exist, so create them as needed. The default values prior to patch 1.2 were:

fMouseHeadingYScale=0.0100
fMouseHeadingXScale=0.0200
However, if you try that now you'll probably find that vertical (Y) movement is severely lacking. You'll have to mess around to find the right ratio. Here is what worked for me:

[Controls]
fMouseHeadingYScale=0.8000
fMouseHeadingXScale=0.0300
This lead to consistent and synced vertical and horizontal mouse movement both at low and high frame rates. It may take some trial and error. If you haven't yet, disable the intro movie so you can get into the game quicker to try out the settings.