Backwards compatibility probably won't be going
anywhere, but the same may not be true of cross-compatibility and
forward compatibility. Knowing which version of Word your client plans on
using still helps you know what *other* formats it can read.
What I was actually thinking was ODF. It'd
solve your problem with RTF not allowing for enough formatting and since it is
an open standard with a fair amount of industry support, there are open source
libraries out there with APIs that PHP can easily hook into. ODF support was not added to MS Word until 2007 SP2
though. No RTM support and no support in previous versions. So it
may not be suitable at all.
OO-XML is another possibility. Office 2007
RTM supports it and there are compatibility packs from MS to add OO-XML to
Office 2003, but 2000 and earlier would be in the dark.
Just a couple of thoughts,
-Cliff