I believe it will launch in the U.S. in spring for several reasons.
1) Sony has been accelerating the U.S. releases of its systems. PSOne was a nine-month wait, PS2 was a six-month wait, and PSP was a three-month wait. Sony wanted to ship PSP in the U.S. simultaneously with Japan, but had to settle for three months.
2) The March PSP launch proves that Sony doesn't feel it's necessary to launch its systems during Christmas season. If Sony could sell 700,000 $250 PSPs in March, they will be able to sell as many $300 PS3s as they can make.
3) Howard Stringer is on record saying he believes Sony should release its videogame systems in the U.S. alongside Japan.
4) The U.S. market is becoming more and more important relative to Japan.
5) E3 is a U.S.-centric show. It's unlikely Sony would release a press release in English at E3 that the system was launching in Spring if the system wasn't coming out in the U.S. until the fall.
6) MS is launching the X-Box 360 in the U.S. in November and Japan in December, so the precedent is set for launching in the U.S. and Japan at the same time.
7) Many of the western developed games like Insomniac's FPS, Killzone 2 and Motor Storm are obviously pretty far into development and could be ready within a year IMO.
8) MS is not SEGA, and X-Box 360 is not Dreamcast. I don't believe they want to give MS a one-year head start.
9) Launching in the spring may give Blu-Ray an insurmountable head start over HD-DVD. Sales of dedicated HD movie players will be slow at start, and Blu-Ray will already be in millions of homes.