While thin clients will have to become more powerful, the reality is that 15 years or so after the creation of the first dedicated 3D display card; business has still not built a strong business case for 3D applications. There simply is no need for main stream 3D SAP/outlook for example.... So old, dumb, thin clients have their place.
For an interesting look at where the technology is heading, check out http://www.citrix.com/English/ps2/products/feature.asp?contentID=1685500
Yeah, low performance dumb terminals would probably be best suited for the traditional TS environment while higher specked machines would be needed for the VDI.