Summerset was never filed iirc, if it was I think we'd see it as a dead trademark. They planned on setting TES III there but chose to go with Morrowind instead. And yeah they let a trademark for Oblivion expire but as you know they eventually used the name for TES IV anyway.
But it does lend some weight to letting Skyrim expire... even if they choose to use it later. The Skyrim application date was a full two years before the release of Fallout 3, and who knows how that really affected Beths plans.
While I see credibility there I hold out hope none-the-less for this timeline:
TESV: ((Skyrim))
3/20/2006 – TESIV released (potential start date for concept artist not used/completed on FO3)
10/10/2006 – Skyrim Trademark Application Date
10/28/2008 – Bethesda resources divided into “next big project” and FO3 DLC
10/14/2009 – The Infernal City Release Date set (Amazon.com retracts “200 years” statement)
11/24/2009 – The Infernal City Released
1/15/2010 – TESting Your Knowledge Trivia Begins (Prize: The Infernal City)
3/30/2010 – TESting Your Knowledge Trivia Ends (Prize: The Infernal City)
4/27/2010 – Bethesda Store Returns with TESIV DLC (Skyrim trademark is for clothing/booklets)
6/2/2010 - elderscrolls.com is updated with an announcement of TESV
6/7/2010 – Cover of GameInformer reveals first screens of TESV (July Edition)
6/14/2010 – Bethesda Online Store includes Official TESV merchandise clothing/booklets
6/15/2010 – E3 begins and Bethesda reveals game play footage of TESV
7/13/2010 - Second Elder Scrolls release date is revealed
7/31/2010 – Skyrim clothing/booklets Trademark Expiration date
8/12/2010 – QuakeCon (possible marketing platform for TESV)
11/23/2010 - Second Elder Scrolls novel is released and release date of 3/15/2011 for TESV is revealed
3/15/2011 – TESV Released
...only 7 days until the announcement! Hopefully...