Windows Marketplace only for Windows Mobile 6.5 and up
Unlike Apple, Microsoft also doesn't make it easy to upgrade from one version of Windows Mobile to the next. It's typically up to the phone manufacturers and wireless carriers to decide whether an upgrade will be available and the upgrade procedure can be cumbersome, to say the least. So odds are if you don't get a new phone running Windows Mobile 6.5 you won't be able to use Windows Marketplace.
That said, this isn't the first controversy surrounding the Windows Marketplace. Microsoft initially told developers that they'd have to pay a fee every time they issued an update for an application listed in the Marketplace. But after receiving strong criticism, Microsoft backed down. Maybe if there's a large enough stink around the Windows Mobile 6.5 requirement, Microsoft will come up with a way to let users with older phones use the Windows Mobile Marketplace.