Well, John Wesley Shipp's Flash wasn't very good, so thats probably not a good example (to be fair, it might have been the writers fault more than the actor, but from what I've seen The Flash tv show was not very good). I'm not going to comment on Ledger, but when it comes to Jackman I think even if I'd never seen him before and they announced him as Wolverine, I'd think it was a good choice.
This Flash is going to need to be a good actor to be a believeable Flash. Plus he's probably too young to be a forensic scientist, and definately looks too young, but they might not even go that route, and if they do its something I can ignore if its written well enough.
Of the live action versions of the Flash I know about (Rod Haase in the Legends of the Superheroes
special, John Wesley Shipp in the TV show, Kenny Johnston in the Justice League TV movie/pilot, and Kyle Gallner as Bart Allen in Smallville
) only Smallville's Flash was anything above average (although I think we can give the Legends of the Superheroes guy a pass, it was just a small roll on a 70's tv special). The TV show one might have been ok with better writing, and the JLA TV movie one was just bad (but everyone was bad in that TV movie, it wasn't just him). Hopefully this new guy can be the first really good Barry Allen. Even with his current acting record and look I don't think a judgement can be made until we actually see him in action. I'm hoping for the best.