Apple is holding back on releasing a new iPhone upgrade; most likely until fall.  The reasons behind this decision are obscured by the Apple shroud of secrecy, but there are some obvious reasons for this based on current market stats.  First and foremost, the iPhone 4 is still a hot product.  Why would they want to cannibalize on existing demand when they (and AT&T) sitting on a shrinking pile of 3GS product, and the iPhone 4 is still in demand. Additionally, Verizon's iPhone 4 is only three months old, and as users shift their feet from AT&T to Verizon, they don't want to deal with hesitant buyers who are holding on the iPhone 4 on Verizon because of the eventuality of the revised model.  In the past 5 years, Apple has typically rolled out version upgrades on their phones during the summer months, but this scheduling most likely changed to allow for the Verizon iPhone 4 to have breathing room to sell.

Also, the next revision of the iPhone is most likely going to be a minor revision year for the iPhone product.  Apple has already decided to not include 4G and LTE antennas on the next iPhone because of the demands of power on the new cellular technology.  The switch to 4G will probably happen in Summer/Fall 2012.  This leaves the model updates to the iPhone to be relegated to increasing the processor speed, possibly upgrading the body design to improve antenna signals and reduce the "Antenna-Gate" issues in the fall of last year, and lastly increase the storage capacity of the phones to double the current storage tiers: 16 to 32, 32 to 64, and 64 to 128 Gigabytes.  The big change that will roll out is the upgraded operating system for the portable devices: iOS 5.