Adreas makes an app which can take almost any kind of subtitle information,
and translate into any other type.
it's called Title Exchange Pro (TEP)
it can also help you out when you need to change a bunch of FCP text generators from one font, say, to another.
the most direct way to "create" DVD subtitles with Andreas's app is to actually use FCP to make up the subs.
you can add subtitle text to markers in FCP, export an XML, and use TEP to make an stl file for DVDSP.
that has it;s disadvantages in that you dont really see your work as you do it
but you can also create subs in FCP's text generators, export an XML, then use TEP.
as far as that goes, i have done it, and the work is much, much faster if you use a macro app like QuicKeys or other to help you out.
there ARE specific subtitling apps out there:
SubBits is one, Vidoe Toolshed make that,
Belle nuit is one,
DVD SP would
not be one!
here are some links from my (old) collection:
[
www.spherico.de]
[
www.belle-nuit.com]
[
www.videotoolshed.com]
[
www.inqscribe.com]
[
www.zeitanker.com]
no recommendations, sorry, i only used FCP with Quickeys and TEP.
cheers,
nick