StealHawk understood me correctly.
What i meant is that writing PS1.3 ( that's GF4, people ) with DX9 should normally be possible ( it might not be possible, but i'd say there's a 99% chance that it's possible )
Well, yes, displacement mapping is another example that won't be implemented that fast since making a DX8 alternative gotta be very painful in some cases. But there are things where making an alternative isn't hard - just don't implement it.
This isn't really a good example since it was possible with DX8 too, but anyway, take shiny surfaces.
If you don't have the graphic power to use them... then just don't. It can simply be an option and there's no problem with that.
Also, keep in mind that you certainly aren't going to see DX9 games ( REAL ones, the ones who actually use the features correctly ) for at least 9-14 months. In a year, DX8 will hopefully become much more mainstream.