This may be the biggest mistake ABC has made since scheduling the infamous "Turn-On".
I think much of "Nightline"'s success, especially of late, is because it's not a comedy-oriented talk/variety show and is in fact something different.
Moving Jimmy Kimmel to 11:30 (ET/PT) will make the late-night talk/variety marketplace even more crowded.
As crowded as the 11:30 P.M. timeslot is for talk shows, it will be more so when Arsenio Hall's comeback begins next year (most stations carrying that show will do so at 11:30 ET/PT); I can even see TBS move Conan O'Brien back a half-hour so his show also begins at 11:30.
This means that come September of 2013, in most cities, Jimmy Kimmel, Jay Leno, David Letterman, Arsenio Hall, Steven Colbert and Conan O'Brien may all be head-to-head, all starting at the same time. Six pretty-much similar shows could divide-up the viewership so much that none is a real "winner" in the ratings.
I don't think Kimmel will do as well at 11:30 as he has at Midnight.
ABC press release: http://www.abcmedianet.com/Web/dnr/dispDNR.aspx?id=082112_01
Wild card: If Governor Romney is elected President, could "Nightline"'s move back to 12:30 A.M. ET/PT be delayed until after the inauguration??