We did have it there for a while, and then arn initially removed it when he upgraded the built-in search engine. I don't know when it returned to the search options, but I feel like it's been back for a little while now.
I'm missing something. The "Search Forums" doesn't appear to use Google, since the search results are different than using MRoogle.
Edit: Nevermind. I see what you're talking about now! Cool! I still prefer MRoogle, since it follows my Google search preferences, such as displaying 100 results per page, while the built-in search bar doesn't.
The Google search option under "Search" has been there for ages.
Totally unbiased of course http://up.*************/files/1/Images/Smilies/Dogeyes.gif, but MRoogle is still better