Good point, Mornington Crescent!
However, I assumed that you could either download and use the same version as Vista:
Download details: Windows Internet Explorer 8 for Windows Vista or Windows Server 2008, or else it would be a case of re-installing from the installation media (Add Windows Components, or whatever it is now in Win7).
I couldn't check these because my Win 7 (RC) PC is offline, awaiting a new PSU.
If neither of the above work, then it's true that you can't re-install IE8 on Win7. Yet!
SD