Preview at WWDC likely in June, followed by September launch.
iPad App Store Reveals Launch Apps, Top Seller Lists, App Details
The fact that the iPad-specific applications are ranked suggests that the applications are being downloaded/purchased at least on a limited basis, despite the lack of iPad availability for the general public. A report yesterday indicated that a number of these iPad-specific applications had temporarily appeared in Apple's web-based 'iTunes Preview' App Store listing.
Early app prices seem to range from Free to $9.99 for the most part, with a few higher priced examples such as OmniGraphSketcher ($14.99) shown here:
It's important to note that all prices and descriptions may not be final as developers can make adjustments to these items at any time. Wi-Fi models of Apple's iPad are scheduled to launch in the U.S. on April 3rd, with iPad-specific applications and services to go live on the same day.