Is it (or will it be) possible to install IIS 8 on Windows Server 2008 (R2) or it's only meant for Windows Server "8"/2012?
Traditionally, it's always been one version of IIS to one operating system.
I see no sign of this changing in the immediate future.
Actually, it is possible to install IIS 8, on Windows 2008, Windows 7 and Windows Vista, see IIS 8 Express Readme. However, this is the Express Edition, which is meant (as TomTom points out below) as an edition for developers.
Not sure whether the OP wanted IIS 8 in a production environment, or just for testing, developing or experimenting. In the latter case, you could just download and install it.
Note that some of the prime features of IIS 8 are not be available on the down-level platforms. Specifically, these are mentioned by Microsoft as not available on platforms before Windows 2012: