Treaties are always open to renegotiation. Been done thousands of times.
However, I do think the "suitcase nuke" scenario is much more likely than ICBM delivery. As towd mentioned, our borders are wide open. You want to worry about something? Worry about THAT threat. Chemical, nuke, biological...anytime they want to I'd suspect.
That notwithstanding, research into ABM technology should continue. The day will come when you can hit a bullet with a bullet. It's what computers are about, isn't it? (Remember when we played DOS AW on a 386SX? Now who'd have projected AH on 1 Gig Athlons at that time?) May take a while, but we'd ALL be foolish <the entire world> to think that sort of technology will never be needed.
Further, the guidance technology/programming for "bullet v bullet" is applicable in a lot of other scenarios, not just against ICBMS.
So, negotiate a new treaty, let research continue and lets not be in such a hurry to deploy anything.