Yes, T3 was pretty awful. But from what I've seen, Salvation looks to do what TV show is doing and reboot the franchise. They've scrapped Arnold, the only remnant from the atrocious third movie seems to be Connor's wife, and the stuff with a machine who thinks he's human is very reminiscent of ground the TV show is currently covering. The original two Terminator movies were so good because they were thoughtful action movies. Like the first Matrix, there was a philosophy behind them that actually encouraged you to question the plot and characters. The third one was more spectacle, just going through the motions and missing the point of the first two in a big way. Yes, to avoid a paradox in the Terminator timeline, war with the machines would have to be inevitable. But it came in such a hollow, meaningless way in T3 that it destroyed the sacrifices of the first two films. From what I've seen of the fourth movie, it looks to care a bit more about the franchise as art instead of just spectacle.
Of course, I'm also a huge fan of the Sarah Connor Chronicles, so it will be interesting to see how the continuity changes through the different mediums. In TSCC, T3 is non-canon. It would be cool if Salvation did that too, but I think they'll have to build off of some of the plot points raised. Hopefully they'll just do it in an interesting way.