How to import KML files into OpenStreetMap?

You can't import KML files directly into OpenStreetMap, but you can probably trace over them in one of the editors available. Merkaartor can open KML files directly, and if you convert the file to GPX JOSM can display it.

Even if you do convert your KML file to GPX, you won't be able to upload it to OpenStreetMap's traces, since it won't contain time stamps. This is done deliberately so that only traces gathered using a GPS receiver, rather than traced from an existing map, can be used as source material. Information traced from existing maps will be derived from those maps and carry their copyright, making them unsuitable to use as a source for OpenStreetMap.

You can also convert it with gpsbabel adding fake time:

#!/bin/bash
for file in *.kml; do 
  gpsbabel -i kml -f "$file" -x track,faketime=20120901000001 -o gpx -F "${file}.gpx"
done

You can load a KML file as a vector background layer in Potlatch and then choose which features from it to merge into the main layer. There is more information in the wiki.

How can one keep some attributes of the KML file when bringing them into openstreetmap? I want to bring in some bus stop data for example, and I'd like to keep the stop names with each stop without retyping them.

I've had some luck with this site, which provides several conversion capabilities.

http://www.gpsies.com/convert.do

If anybody has any warnings about this source, please speak up.