All these projects are in 'pre-production'. What this means is that people have talked about maybe making a movie. Perhaps some actors have been sounded out. Maybe even a script is being laboured over somewhere. But only a small proportion of films that get announced as in pre-production ever actually get made. As Gideon Nisbet says, advance publicity is 'so much hot air until someone starts nailing a set together'. So, Variety may announce that Zak Penn, writer of X-Men 3 and The Incredible Hulk, has signed with Twentieth-Century Fox as writer and producer of The Argonauts, but that doesn't mean that they are committed to putting serious money behind it, however much the publicity department may talk as if this is the case. Reading between the lines, it looks like this is a pet project of Penn's, that he's got some money out of Fox to write a script for. What will become of it depends on a variety of different, and unpredictable factors, not all of them relating to quality.
Last year, for instance, there was much talk of a film of Robert Harris' novel Pompeii, to be directed by Roman Polanski. Plans were afoot to begin filming in Italy, with Orlando Bloom and Scarlett Johansson 'in talks' (another term which, like 'pre-production', covers a multitude of sins) to star. Then the project was delayed due to the possibility of a strike by the Screen Actors Guild, Polanski couldn't commit to the revised schedule, and various distributors pulled out. No new director has been assigned since Polanski left, and though the film still appears on the Internet Movie Database, it seems to me not unreasonable to assume that the project is dead in the water.
What's happening at the moment is that the success of 300 last year has encouraged studios to look at more similar ideas, in the hope of repeating that film's success. The present vogue for films adapted from comic books is also a factor; Hercules: The Thracian Wars is a comic that has been optioned. But this is just a cycle that comes and goes. People talked up the epic when Gladiator was a hit, then talked it down again when Alexander flopped. If Watchmen tanks, comic book films may go out of fashion.
So, I don't expect to see most of the films that have been announced. Some I'm sure will never happen. Vin Diesel has been trying to get his Hannibal the Conqueror since at least 2002. No-one seems interested (Gideon Nisbet has an interesting examination of why this might be in Ancient Greece in Film and Popular Culture), and I certainly don't believe IMDb's suggestion that it will get a release in 2009, when not a frame of film seems to have yet been shot. Even the animated prequel, which at one point had its own webpage, suggesting it might really happen, seems to have gone into limbo.
Of all these films, Boorman's Hadrian has the most chance of actually appearing. It's got a name director, a big name star in talks (Daniel Craig, or is is Antonio Banderas? Personally I'd like to see Peirce Brosnan in the role, but that's just me, I guess), and a schedule to start filming next spring. But it's currently no more solid a prospect than Pompeii was this time last year, just before it all fell apart. For a film to get made requires not just the allocation of a budget, but some serious spending of it, not just on rights and scripts (relatively cheap in the overall scheme of things), but on locations, and sets and actors.
Once that investment starts, a film can survive all sorts of disasters, and usually (though not always) will make it to the screen. Gladiator's second script got thrown out just before filming started, and Oliver Reed died before completing his scenes, and that still got to the multiplexes. Of course, sometimes it takes a while, if the execs are worried that their project isn't any good; The Last Legion was delayed by over a year.
I'd love to be able to see a classically-based film in the cinema about every other month over the next two years. The Hadrian and Claudius pics have the potential to be classy pieces of work. But until the cameras start rolling, I'm not holding my breath.