Verbal - I think most LLs keep some sort of "partner/preferred" list - we just dont advertise too much about it except by private requests. There are so many things you can do with submits before they get to the review que with IP/Whois checking/dupes and multisubmitter checks - that we rarely get over 200 submits a day that make it into the que - after that its a matter of getting it pared down based on content, quality and all the other things that reviewers get paid to do
I would say that with the recent sucesses of some LLs with partner accounts, and the increasing fraud and autosubmissions in the last few years, it wont be too long before most LLs get to that point