Hannah:
I wish there was a keyboard shortcut to open system preferences.
Tantek:
That would be useful, wouldn't it. ... Your wish is granted.
On Mac OS X 10.6 (likely works on previous versions)
Now no matter what application you're in, if you simply hold down the shift key and press command-comma (the normal application preferences shortcut), it will open System Preferences instead.
Afterwards I found this article on Macworld which explains pretty much the same thing in prose, however they recommend command-option-dash as a shortcut.
I prefer shift-command-comma for the System Preferences... keyboard shortcut because my muscle memory is already trained to think command-comma for the application Preferences menu item, and it's easier to just think "shift" (like shift-up to the level of the whole system) for system-wide preferences.
Update: On Mac OS X before 10.6, you may need to type in an actual ellipsis character (…) in step 7 System Preferences… (via @ljharb: 1, 2).