As covered in the Apple Q2 2009 conference call, Apple Chief Financial Officer, Peter Oppenheimer, repeated the company line that they "looked forward to [Steve Jobs'] return at the end of June".
Apple's World Wide Developers Conference (WWDC) -- where in 2008 Steve Jobs unveiled the iPhone 3G to the world -- takes place this year June 8-12, which is decidedly before the end of June, and thus the end of Jobs' leave of absence.
Does that make it impossible for Steve Jobs to pull out a surprise appearance, iPhone the Third Generation in pocket? No... but it makes it unlikely.
That leaves Apple marketing honcho Phil Schiller, who handled Macworld 2009, or iPhone marketing head Greg "Joz" Joswiak, who handled part of the iPhone 3.0 Sneak Peek as the likely suspects, doesn't it? (Perhaps with Scott Forstall again on the 3.0 software?)