I missed the original announcement, but it seems that Marvel stated in 2008 they would be reprinting their amazing Masterworks series in trade paperback form, to celebrate their 70 years of existence in 2009! They'll be released at the rate of one a month in the same sequence as the original hardcover printing, starting w/ 'The Amazing Spider-Man: Vol. 1' which collects his debut in Amazing Fantasy #15 and The Amazing Spider-Man #1-10.
Marvel Masterworks actually started in 1987, where they would collect various series in chronological order into mostly hardcover and a few trade paperback, full-color reprints. It kind of petered out in 1994, but they ended up bringing back the hardcovers, w/ a new cover format, a few years later.
During the original run, between 1992-1993, they published lower-priced paperback editions of selected volumes in the line, but they only reprinted half of the contents compared to the hardcover editions. They also briefly partnered with Barnes & Noble in 2002 to produce twelve lower-priced TPB versions of selected Masterworks volumes, but (until now) no trades were being done. I've got a smattering of each for the Avengers/Fantastic 4 and a hardcover for X-Men Volume 1...so i'm stoked to finally be able to put together an entire set.
Spidey was just released last week and The Fantastic Four, The X-Men and The Avengers will follow each month hereafter at only $25 a pop. This is a great opportunity to add some Lee, Kirby and Ditko's best work to your bookshelf...amazing reading whether you are a comic head or not.