There's a new search engine referral URL as part of Twitter's new interface. You'll want to update those search engine configurations to be on the lookout for it, and note the additional parameters too. Here's what the referring URL looks like from their new sidebar search:
I see one other category in there for the new "trends" they've added just below the search bar, but in my limited testing, the referring URL didn't always reflect the "trend" selected, which I hope they fix.
I also note that if you've searched for something, that search phrase stays in the URL, even if you go somewhere else on the site (example: I went to my @elbpdx area and clicked on a link in there and the /search?=searchphrase was still present in the referrer...I imagine this is a bug that they'll fix...)
Expect more changes
The URL portends some additional goodies that they must have up their sleeves. What additional "source" items will we be seeing? What else are they going to do with the "category" field? I'm looking forward to seeing more...