Microsoft is making a first developer preview of IE 9 available at IETestDrive.com. It is a 31MB download and will run only in Windows 7, Windows Vista Service Pack 2 (SP2) and Windows Server R2. This is not a full-featured browser by any means, but includes the underlying rendering engine and should at least offer a first look at the work Microsoft has done so far. There's no navigation buttons or built-in security, for example.
The development team is committing to delivering an update to the browser code every eight weeks, putting the first such update in mid-May, and to interact with developers via the Microsoft Connect feedback loop as the company moves toward a more full featured beta. Unfortunately, it remains unclear when a public beta would be ready.