I just performed an upgrade from my Vista Enterprise SP1 x64 bit to Windows 7 build 7000 on my Lenovo T500. The upgrade path from Enterprise edition to Ultimate is not supported but with a quick little registry fix it turned out to be a painless upgrade.

Prior to the upgrade change these settings:

HKLM\Software\Microsoft\Windows NT\Currentversion\EditionID (value Enterprise) to Ultimate

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This should be enough to pass the prerequisite check and start the upgrade process.