If Apple is going to hold an event, it will be for the iPad, not for 4.3. The iPad2 may SHIP with 4.3- which could contain some specific minor features, but I'd bet heavily against it being a key part of the event. Why?
Because iOS 5 is just around the corner. Let's look at the past 3 years:
iPhone OS SDK Preview Event, MARCH 6, 2008
OS 3.0 Preview Event, MARCH 17, 2009
OS 4.0 Preview Event, APRIL 8, 2010
We aren't far off from this years iOS 5 Preview for...