The service will go live for both the PS3 and the PSP at 1am this Thursday morning May 20. From launch there will be the ability to rent or purchase one of 600 movies from all the major studios, with at least 50 new movies being added every month.
The service differs from the Xbox's Zune Video Marketplace by offering movies to download (with price starting at $7.99) and by focussing on the download aspect rather than HD streaming. Even rentals (available in both HD and SD and starting at $3.99) are downloads rather than streams, although apparently there are "progressive download technologies" which allow you to start watching the film before its finished downloading.
The rentals have the standard "14 days to start watching, 48 hours to finish watching once you start" conditions associated with this kind of service. Although this offering does have the added benefit of being able to transfer movies from the PS3 to the PSP if you're signed into the same PSN account on both devices.
It's pretty exciting stuff, although you might want to upgrade to an unmetered ADSL2+ plan before you start watching - HD downloads will chew through your data pretty damn fast...