Merrick here...

A few weeks ago we learned of James Cameron's plans to hold free screenings of AVATAR footage in IMAX theaters across the country. Even though this event is free, tickets are still be needed to get into these screenings - Fox & Co. have now released further details regarding where and when to nab tickets to the 16 minute preview.

Per THIS piece in the L.A. Times:

On Monday, 20th Century Fox will launch an unusual offer on the website for its Cameron-directed film "Avatar": free tickets for an early look at 16 minutes of footage from the futuristic thriller that will be shown in more than 100 Imax 3-D theaters around the world.

With two screenings on Friday, Aug. 21 (at 6 and 6:30 p.m.), the "Avatar" preview will include an introduction from Cameron and some new footage not shown during July's Comic-Con International convention in San Diego.
[EDIT]
In what’s sure to be a mad grab among sci-fi fans, tickets will be given away a first-come, first-served basis on the “Avatar” website (www.avatarmovie.com) at noon PST Monday.

It should be noted that not all towns/cities/locations with IMAX screens will be hosting this screening. In Austin, for example, the local 3D IMAX will not be hosting this AVATAR preview because it did not book the film for the Christmas season (opting instead to present Robert Zemeckis' A CHRISTMAS CAROL in the IMAX 3D format over the Holidays).

To those luck enough to get in: we'd love some reports about your experiences, and detailed perceptions about what you see.

AVATAR's trailer will also emerge on August 21:

On the same date, theaters worldwide will begin showing more conventional 2D and 3D "Avatar" trailers.
[EDIT]
...will continue to play in coming weeks before feature presentations of Fox Searchlight's comedy "Post Grad," the Weinstein Co.'s Brad Pitt starrer "Inglourious Basterds" and other late summer and fall releases.

...says THR HERE!