Its all a guess but Apple rarely lets a product go for more than a year without a major update. We have had a minor update to the 16gb so anytime from June on increases the chance for a new iPhone.
Its the way with technology, there is always something better around the corner. If you want an iPhone, just buy it. You can always buy a new one later and eBay the old one.
That's my biggest concern, but do you really think that they would set the price at 599 again after the price cut fiasco last year? I just think raising the price again would scare people away from buying it sooner than later because of the huge price cuts could happen.