I'm guessing we won't be seeing iPhone OS 4.0 on iPad anytime soon. Apple releases SDK beta's so developers can test the OS in combination with their applications. Apple won't ship the iPad with an iPhone OS that hasn't been tested by the general developer public. So no OS 4.0 for iPad.
Last year, Apple previewed iPhone OS 3.0 in March (17th to be exact) at a small special event on their campus. I would say they'd use a similar timeframe and venue this year, but since the iPad is being released in April, this event might have been postponed to sometime next month.
I've already heard some very quiet rumors of an event in the third week of March. But because a very odd date (Sunday, March 14th) was mentioned, you can't take it seriously actually. Still, there might be an event coming, next week perhaps not but at the very least in April. Apple definitely wants to preview their new iPhone OS/SDK to get people hyped up for the new OS and the new 'A+' iPhone.
For what it's worth. Prior to Apple's iPad event there were some rumors Apple would rename iPhone OS to 'iOS'. As we all know they didn't, but I think it's still possible. iPhone OS refers to iPhone, but since both iPod touch and iPad also run iPhone OS that name won't cover the general use of it anymore. A new name would be the solution. Though you could rename it even to iPod touch OS and iPad OS of course. Just a guess.