Simulator-only apps developed with the iPhone SDK 3.2 Beta 5 can be submitted as of today for initial review. Upload your apps by 5pm, Saturday, March 27th, and the App review team will e-mail you with submission feedback about the readiness of your application for App Store distribution. You will also receive information about submitting your apps for final review, before the iPad ships and (for most of us) before we even own hardware.
If you're thinking about waiting: don't. Apple states that "[o]nly apps submitted for the initial review will be considered for the grand opening of the iPad App Store." An Apple spokesman further confirmed that "[W]e are looking forward to having an amazing line up of apps available when the iPad ships on April 3." The iPad App Store will launch at the same time as the iPad device.