I know this is way late, but I was reading this question/answer combo and though of an alternative method that doesn't require an image. I threw together a post on my blog explaining. I figure it would be helpful to future visitors to have both of these answers handy here.
http://web.archive.org/web/20120520203231/http://www.anotherandroidblog.com/2011/06/29/drop-shadow-linearlayout/
