I know that if I install Exchange 2007 (with or without SP1) on Server 2003, I can not do an inplace upgrade to Server 2008.However, if I install Exchange 2007 SP1 on Server 2008 RC1 today, would an inplace upgrade to Server 2008 RTM be supported?Thanks

Its best not to rush it. I'm eager for 2k8 to rtm as well... here explains what you already know, the fact you can't upgrade 2k3 to 2k8 with exchange http://msexchangeteam.com/archive/2007/10/04/447188.aspx you can, in fact upgrade 2k8 rc to 2k8 rtm. with or with out exchange, i dont has been officially posted anywhere yet. You probably can, but again, dont rush it.

I've just done exactly that and have blogged the procedure here: http://blog.solarfusion.co.uk/2008/02/upgrading-server-2008-rc-server-running.html Basically you need to re-install the CAS role after the "upgrade" to RTM. Alex