Any intentions to build that into 9?
As always, the answer is sure, if that is something people really want.
As 9 expands a general interest is consuming styles that other packages have created. That helps to migrate data into 9 and it also helps generate richer libraries of cartographic symbology. Why not recycle what is available, right? You can see that today in the use of fonts as symbols. That immediately adds hundreds of thousands, if not millions, of symbols that can be used for points.
The issue with any such thing, of course, is the extent to which people actually use it, the extent to which supporting any such "standard" provides access to a significant library of symbology that can be recycled, and to the extent such symbology is reasonably compatible with what could be done better in 9.
Some of the latter depends on a balance of technologies in 9 that are planned, in the works, awaiting integration or otherwise not yet in public builds. An example of that is web serving in 9. Suppose that is far easier and better than, say, MapServer. In that case it might be a better use of resources to arrange an import converter of SLD intended for MapServer so the styles can be migrated into 9, more so than to arrange an export/converter that might convert a richer superset of 9 styles into SLDs that could be consumed by MapServer.
I'm not taking a position on this, just exploring some of the issues. Some other issues that might be considered are in the discussion of "new formats" in the Suggestions page.
It's a case where the user community can and should express opinions and contribute expertise. There are some things, like GML, which in theory sound great in some cases but which in practice are wretchedly full of wrongness. The way to tell is to tap into the collective expertise of the community. If there was a universal way of describing styles so all the world's GIS peoples could be one happy family, Manifold surely would be wise to adopt it. Is SLD that universal way?