It is officially a done deal per Twitter, the Summize logo has already vanished, and the URL redirects as well. The community was correct. --The Industry Standard

This prediction may not be open for long. Techcrunch and others have it that Twitter is on the verge of buying Summize:

"The deal certainly makes sense. Unlike rival Friendfeed, Twitter still lacks a search feature. And Twitter has relied on Summize in the recent past to help reduce load on the Twitter API. Summize is also one of (or the only) Twitter partner that has access to their XMPP stream."

This is a prediction that Twitter will announce a deal to acquire Summize. The deal need not yet have gone through; the announcement of an agreement needs only be made public.