Mick Foley not appearing at Summerslam was purely a Vince McMahon call. There were plans to have Foley involved in the match, partly to play off of his angle with Edge and partly to play off of the 10th anniversary of his 1998 King of the Ring Hell in a Cell match with Undertaker. Vince simply told the creative team that they were moving in a new direction without Foley.
A source of within TNA said today that TNA is already working on Mick Foley t-shirts and other merchandise ideas. This would seem to indicate that Foley is headed to TNA.
Mick Foley’s management agency, The Gillespie Agency, is expected to make an announcement about Foley’s future on 9/1 (which is after his WWE deal expires). Most seem to think that Foley will jump to TNA.