At time of writing, this hasn't been reported anywhere, but I expect to hit (officially) in the next few days - or maybe just hours.
Sin City 2 has been postponed indefinitely and insiders close to the project suggest it may not go ahead at all, at any time.
Rumours have begun surfacing that Angelina Jolie is pregnant again (possibly all spawned by the issue of Star magazine that is pictured above), and indeed this may be a contributing factor. Apparently, Robert Rodriguez was holding off for her, wishing to cast her in the key role of Ava Lord, the 'Dame to Kill For'. Supposing she has recently let the production know that she won't be available, the sudden shut down seems like quite a logical course of action, at least to regroup and consider the next move.
Other tattletales suggest that Rodriguez has just grown tired of the whole Sin City thing, or more pointedly, has suffered so many disagreements with Frank Miller - who can reputedly be rather arrogant about and controlling of his creation - that Sin City isn't anywhere he wants to visit again. Is it possible that Rodriquez will extricate himself from the production and eventually, a second Sin City movie will be direct by Frank Miller alone?
I'm told that some of the cast, still believing the project is a go, have been speaking to Frank recently... but not Robert. Simply because Robert is busy with Grind House? Or could there be a little more to it?
I've got feelers out for better information, and I'll bring the details to the front page of film ick as soon as I hear a single thing.
[EDIT: Rich Johnston at Lying in the Gutters has now chipped in with his findings. They're not the same as mine at all, and make for very interesting reading. All the same, I've had even more reports since this post went up that seem to corroborate the original story. Only time will tell exactly what is going on here]