You can get an evaluation copy here.

Scott Schnoll had a great post on how to install the beta, with all the gotchas and a long list of pre-reqs.

http://blogs.technet.com/scottschnoll/archive/2009/04/15/how-to-install-exchange-server-2010.aspx

As Exchange 2010 will only run on Windows Server 2008 (64bit only), there were some known issues with the beta version with the Windows Server 2008 R2 (mainly newer builds than 7000), due to PowerShell and WinRM stacks being incompatible. That issues is well discussed here and hopefully those issues are now resolved with the RC.

http://social.technet.microsoft.com/Forums/en-US/exchange2010/thread/e73ec63f-d5f6-4c2d-8d96-51537493a0ff

And here is the system requirements list.

http://technet.microsoft.com/en-us//library/aa996719(EXCHG.140).aspx