I read recently that a couple of B-2 stealth bombers have been relocated to the base closest to Iran that has the necessary facilities to handle B-2 needs (it's in England). This could be routine, or it could be that the flight crews are starting to get familiar with flying into and out of that base so that they can be ready to go when/if we attack Iran. Since it's doubtful that we'd use ground forces in Iran, and would instead just use our air and naval superiority against them, there won't be a whole lot of indicators of an impending attack (like giant ships traversing the oceans laden with tanks and troops), but B-2 relocation is potentially one indicator that preparations are now underway. Another one would be carrier groups edging closer to Iran, which so far as I know isn't happening yet, but since that can happen in a matter of days, that's something they'd leave until the final week or two.

If we are going to attack Iran, it'll probably happen in October, shortly before the mid-term elections. This would allow Bush, Inc. to televise footage of obliterated nuclear research facilities and the like, which would probably help the Repugnicans do better in November.