This somewhat helps with a problem they are facing which is fragmentation of questions and answers across their sites. For example, for Ubuntu you might need to search three or four of their sites before finding an answer to an issue you are having.
Name : [so] Search StackOverflow // the "[so]" here is just a mnemonic
Location : http://www.google.com/search?q=site%3Astackoverflow.com+%s
Keyword : so
Then, when I want to search for "vim vs emacs" on SO, I open a new tab (Ctrl + T), type "so vim vs emacs", and press enter. This searches "site:stackoverflow.com vim vs emacs" on Google.I have a similar smart bookmarks setup for HN searches, and you could get more creative if you want (DuckDuckGo search, or even a custom StackExchange search like rayvega mentioned, using "http://stackexchange.com/search?q=%s in the location field).
Chrome has something similar: http://www.google.com/support/chrome/bin/answer.py?hl=en&...
This works automatically with any site you perform a search on, with no need to configure anything. One of the main reasons I moved to Chrome after years of being an FF user.