Have you seen Motoi Yamamoto's salt 'paintings'?
This article is has some wonderful photos and discusses why he started working in this painstaking and very temporary medium. (Yamamoto's blog is great too.)
I tend to consider finished things very precious. Things I make, things others make. Artists whose work involves temporary installations what will most definitely be dismantled, painted over or swept up are fascinating to me. There must be a little bit of sadness knowing that something you put so much love into will be washed away at the end of the month or year. It makes my heart flutter around a little just thinking about it! But maybe that short lifespan makes it MORE precious...especially for those who had the chance to see it when it was around.