The last issue of THE TWELVE was issue #8, which shipped in October 2008. That was also when Marvel released a HC of the first 6 issues of the series.
As for what has happened? Who knows. Artist Chris Weston hasn't updated his blog in a while. JMS has since signed an exclusive deal with DC and they originally tagged him to write the going-nowhere BRAVE AND THE BOLD, although as that book's sales are skidding, there is some rumbling that DC is rethinking that plan, as JMS is one of few A-list writers left (his perennially late THOR always sells a good 70-80k an issue). The book hasn't been solicted in March or April, and as noted, is running about two months behind, at least (and it had been a bit late by issue #8, since the first issue was in Jan. 2008).
It would not be the first mini that Marvel has commissioned that either saw a large delay of months or even years before being finished (ULTIMATE WOLVERINE VS. HULK, Kevin Smith's BLACK CAT mini, Joe "Who's the Boss?" Queseda's two mini's, NYX or DAREDEVIL: FATHER), or, if never finished, the first mini that never was finished (Kevin Smith's DAREDEVIL: TARGET only shipped one issue years ago).
Still, Weston's art is detailed, and it could be that he simply fell behind. Weston and JMS had a bit of lead in time before the first issue shipped, and the book started facing delays after several months anyway. A 2-3 month delay for a detailed artist is hardly uncommon; look at Hitch's delays for ULTIMATES 2 by the end, which seemed to take 4-6 months an issue.
Aside for a cover for THE TWELVE #9 featuring Blue Blade, it hasn't been solicted this year yet.
http://comicbookcatacombs.blogspot.com/2009/01/hey-marvel-where-heck-is-twelve-9.html
It is a shame, for I did enjoy the series. Usually JMS is a slow paced writer, but with the book's large cast, that wasn't a burden. It would be a shame for it to go unfinished.