Between convention appearances, Turbine's Sapience confirmed on the forums that while Isengard players won't be enjoying any additional three-man or six-man dungeons, more will come by the end of the year in Update 5:
We are currently preparing five new instances for release post-Isengard.Rise of Isengard will be on display at Turbine's booth this weekend at PAX in Seattle.
Rise of Isengard will be released on September 27 along with a 24-player raid. There will be 3 three-player, 1 six-player and 1 twelve-player instance released in Update 5, the first update after the release of Rise of Isengard. These instances will be free to any player who has pre-purchased the Rise of Isengard expansion or who purchases after launch via the Turbine web store.
Update 5 will be available before the end of the year.