Consider this context: under Windows, start can generally be trusted to attempt some useful "launch the application/document/item/...", very much as command-line open does under Mac OS X.
At a policy level, one also often hears, "Well, there are good standards for installing an application under Windows (or Mac OS X), but Unix (or Linux) is 'all over the map'. Unix needs a framework so we don't have to develop a separater installer/... for each flavor."
The Portland project [L1 ] aims to fulfill that need.
At least, it does, very broadly. While several bright people [provide references] are enthusiastic about Portland, CL regards its ambitions as severely limited. Even when the first couple of releases of Portland are complete, it will leave many, MANY questions of interoperability unanswered. 2007 might end with Portland only creeping past its x86-Linux GNOME-KDE base.
On another hand, Portland has good people working on it, it seems to be meeting its modest deadlines, and it does help with several specific technical issues. For example, xdg-open [L2 ] is close to the start mentioned previously.
[Mention that, while the command-line interfaces are likely to continue to be of most interest to the Tcl crowd, C bindings will eventually appear.]