An update on yesterday's post.
Hugh says it is probably Deroceras reticulatum, the Grey Field Slug. It looks about right to me.
I found a site with good photos and descriptions of slugs of this end of the continent, from Oregon State University.
It seems, from Hugh's post, and from OSU, that there are more varieties of slug around here than I thought. I'll have to look at any I find more carefully in the future.