Announced over the weekend was a book from IDW entitled Walter Simonson's Thor: Artist's Edition reprinting all of the original art from Thor #337 from Marvel. I suspect the initial thought from many people upon hearing about this was, "Wha-huh?" So let me throw out a few vaguely related thoughts that I can't really seem to flow in a cohesive narrative right now...
Marvel actually has a history of letting other people use it's characters dating back to the 1940s Captain America serials. So that's nothing new.
Marvel has also worked with other comic publishers to develop new comic books. Even if you leave aside the one-off team-up specials between Marvel and DC, Marvel also farmed out four of its core titles in the mid-1990s to Image. (That would be the "Heroes Reborn" books.)
Marvel has also has a history of letting other comic publishers reprint their earlier work. Most notable is Dark Horse's reprinting of Marvel's original Conan the Barbarian series.
IDW, despite being a comic book publisher, isn't really a direct competitor with Marvel. The books they sell, generally, don't overlap much in terms of theme, tone or audience.
Marvel has repeatedly licensed the use of its characters in comics that, while published by Marvel, were created for use in specific channels. The Taco Bell comics are probably the most recent example there.
Sometime around 1999, Marvel switched from being a comic book publisher to a character licensing company. These days comics comprise less than a third of Marvel's overall revenue and they make most of their money from letting people OTHER than Marvel do stuff with their characters. Though this is typically in the form single images -- shirts, toys, games, candy, etc.
A little over a year ago, Marvel was bought by Disney. Currently, Disney farms out character licenses for use in publishing comics to Slave Labor and BOOM! Studios, despite some speculation at the time of the acquisition that those titles would be handed over to Marvel.
Various TwoMorrows magazines (most notably Jack Kirby Collector) publish large chunks of original art from Marvel books. In fact, they published Fantastic Four #108 in its entirety in JKC #53... which led to Marvel getting Stan Lee to re-script it and publish it themselves!
All that said, I'm not surprised Marvel is letting IDW have a crack at reprinting their old work. (Albeit, in a slightly different than the original form.) I suspect we'll be seeing more of those types of projects as time goes on. Assuming, of course, this first one is profitable! I'll be curious to see, if this is successful, how Marvel moves forward and whether they'll start licensing their characters to other publishers on a more regular basis.