In an interview with Vaneta Rogers at Newsarama, Senior DC VPs Bob Wayne and John Rood announced that both Batman and Detective Comics would become $3.99 books in 2012, joining Action Comics, Men Of War, Justice League and All Star Western, with added pages in return for the extra dollar.
Usually this means 28 pages of story, a 20 page story with an 8 page backup strip, or 20-22 pages of story with some extra text material, exploring the presented world or talking about upcoming comics.
Bob Wayne says "The reason for adding the story pages is for the story and the editorial content. I think that when you find out what we're doing with those extra story pages, it will all make perfectly good sense."
If I was a betting man, I'd say that there will be a back up strip for both titles, that will provide one story between them.
They also announced digital/print combo packs for Green Lantern and Detective Comics, joining Justice League, Batman and Action Comics. The digital combo price of Green Lantern will be $3.99, Detective Comics will be $4.99 and Batman will increase to $4.99.
UPDATE: Batman writer Scott Snyder confirms they are back up strips, writing "On new backups in BATMAN – look for an announcement on DC Comics blog, the Source on Monday!"