It might be a long wait for the full lineup to be announced for Summerfest 2007. I usually check pollstar.com for the most up to the minute concert announcements. I have found that they always have listings up prior to Ticketmaster.
There have been a few announcements made for some of the side stages; which on the Summerfest grounds can still garner a significant crowd. Acts such as, Phil Vassar, Heart, Buddy Guy, Foreigner, Randy Travis, Umphrey's McGee, Sugarland, George Thorogood & The Destroyers each have dates on the Summerfest calender.