In my opinion, I think the longer Apple waits to announce the launch date of the 3G the more it hurts their bottom line. How many posts have we read from people who were planning to pick up a 3g but elected to pick up the less expensive Wi-fi model because it was available. Now, I'm not naive to think that some of those Wi-fi buyers won' return their purchase when 3g becomes available. But there will be some of those wi-fi buyers who simply elect to keep their purchase. So Apple loses potential revenues of $100+ per unit because folks did not want to wait for the 3g model.
Worse, you have to imagine a number of folks are making "impulse" Ipad purchases. They go into Best Buy, play around with Ipad and decide "it's cool....I just have to have one". Well in my mind, Apple needs to strike while the iron is hot...get those 3g units out there quickly. If you give potential customers a perceived long, drawn out, waiting period. They may decide to save their discretionary income and/or become frustrated and decide to forgo purchasing an Ipad.
At the end of the day I'm betting that Apple gets the 3g out by the end of April. But I think it would be in Apple's best interest to announce a date ASAP. Even if the date has slipped into May, let everyone know now. The longer you make folks wait, the less tolerant they will be if the shipping date does slip.