By the way, you might have missed it at Google I/O, but the Custom Search Engine team has made it really easy to add custom search to any site. Google recently introduced Web Elements, which are simple snippets of code you can copy/paste into your site’s HTML.
From the Custom Search Element web page, I copied the code. Then in the WordPress control panel under Appearance->Widgets, I clicked to add a new “Text” widget, changed the title to “Search (CSE)” and pasted the code into the box:
Click “Done” and then “Save Changes” and that’s it! No need to register, sign up for anything, get a user ID, or anything like that. You can see the result on the right-hand sidebar of my blog.