Bink has some exclusive info on the last stages of Windows 2003.
ESCROW is final stage where Microsoft deploys the build on their own production servers
Windows Server 2003 Build 3757
This build was NOT meant to be published, but Microsoft Dev Team badly wanted to release a new feature for public testing - "IE Hardening"
Basically, it's a policy that you set for the server, and it makes Internet Explorer only reach sites listed in "Trusted Sites".
Windows Server 2003 Build 3760
Targeted for ESCROW, this was first targeted as "the final RC build". A few problems surfaced and id did not make it.
Windows Server 2003 Build 3763
New build targeted for ESCROW, to be "final RC build" to go public. Build 3763 is the last one to have public BETA RC PID.
Starting with Build 3764 real PID's apply. Old BETA-keys do not apply.
News source: Bink