I wasn't aware that Bale had signed to play the lead.
I posted a week or two ago that this movie was slated for the summer of 2009 but, at the time, no definite release date was announced, nor was any castings as reported on scifimoviepage.com.
Since I posted that last comment about no big stars, I do see that Bale is listed on IMDB as playing John Connor, but I'm not sure if that's official yet or not.
Yes, I'd definitely consider Bale a bigger star than Nimoy (or anybody else associated with this Trek movie, for that matter), and the new Batman movie can only make him bigger. While I have certainly enjoyed Nimoy's work on Trek over the last 40 years, let's be honest, he hasn't done much else. Most people know who he is, but Trek is all they really know him for. Quite honestly, Shatner is a much bigger name, although he also is best known for Trek.