This is pure speculation, but.....
What if the new 3G hardware design is ready to roll? Maybe the small delay at the moment is the time it takes to change the production lines over to incorporate the 3G antenna & transmission circuitry.
They could quietly put the 3G stuff in the existing model, keep selling it as an EDGE phone, with the chipset permanently switched down to EDGE by software control (most 3G chipsets will run EDGE - my Palm Treo defaults to it if no 3G signal can be found) and when they announce the big official 3G launch at WWDC (to coincide with the iPhone 2.0 software) Steve can pull one of his big surprises - "Oh, and by the way - every iPhone sold since April is ready for 3G with just a software update."
Steve does like to surprise people. Just a thought.
Aside from the one guy who already said that any idiot could open up a new iPhone and see the chipset, I have another objection to this thought.
They wouldn't release a 3G iPhone under the radar, sell it at normal price, and then give out a software upgrade later to activate it. They would lose a ton of money that way. Even if they pulled the X-amount of dollars for the software upgrade, it would be a terrible marketing technique. That would be like a Christmas miracle and it's just not going to happen, because Santa isn't real.
Sorry bud, 3G for free... not happening.