Bush/Cheney have 15 months left in office. I believe that W wants to leave a legacy of war behind, and as a part of this, he has plans to make an incursion into Iran, most likely in the form of bombs on the nuclear processing facilities and other "strategic targets". The idea is that will force the US to be involved in a war in the middle east with Iraq, Iran, and Afghanistan being the center of it all.
How this will happen? The administration will be pushing more and more the notion that Iran is a "breeding ground" for terrorists, and that as we get further and further, more of Iran's organizations will be associated with terrorism, if not declared terrorist organizations outright.
I believe this will happen before the next election, and I believe that part of it will be decided because the Bush/Cheney regime don't believe that a diplomatic solution is possible and want to assure that diplomatic avenues are closed before the next president comes into office.
This is purely speculation on my part, and not making a judgment as to whether it is the right or wrong thing to do. Timing, I'd say within the next year, probably in the first half of 2008.
And just in case, here is the nuclear survival manual.