I vaguely recall seeing some 4E Dungeon Eberron articles along these lines...
...here we go: Taer La Shala, the Fortress of Fading Dreams. The place itself is real, but seems to have many dream elements that tie into it, and features an item (Philter of the Fading Dream) which results in the PCs exlporing the place in their dreams.
(Part 1, Part 2, Part 3)
Other potentially useful content:
-Sleeper in the Tomb of Dreams