That's a little misleading. You don't really "target" the 3.0 or 3.5
frameworks in the same sense that you "target" the 1.0, 1.1 or 2.0
frameworks. You can, however, use the 3.0 framework by installing the 3.0
framework and adding the extensions to VS as you mentioned.
I was referring to the capability in VS2008 to let you denote which
framework you are targeting. My presumption is that if you target .Net 2.0,
you can not compile a project with .Net 3.0 components in it.
But when you install the 3.0 Framework and the VS extenstions for WCF and
WPF into VS 2005, you are still working against the 2.0 CLR (framework) but
using 3.0 components. So, you can, in fact, do exactly what you presume you
can't do.
That's my point, the 3.x Framework sit on top of the 2.0 Framework and so
you don't "target" the 3.x frameworks like you do the others.