First saw this because Jason Massie twittered about it. But apparently there are enough bits of the .NET Framework in R2 Core where SQL Server installations will be possible:
Andrew Fryer's Blog: Windows Server 2008 r2
It'll be a command-line install, since core has no GUI, but that's fine and dandy for me. One of my biggest gripes with Windows Server 2008 Core when it came out was that although the stripped down OS was ideal for a SQL Server install, because you didn't have .NET (and couldn't install it), you didn't have the ability to install SQL Server. The same was true of ASP.NET based web sites. I don't know if the ASP.NET web sites status has changed, but I'm glad to see that SQL Server has.