Graceful degradation/progressive enhancement for ActionMailer templates?

Is there some gem or technique that will let us write only .html.erb
templates for our Rails 3 mailers, and gracefully degrade them by
stripping HTML tags for the text/plain version, rather than having to
create each partial twice?

Google is seriously failing me, so I must be searching for the wrong
terms; it's hard to believe nobody's ever even asked about this before.

Something i've tried in the past is write the email templates in
markdown (i.e. a template language which is designed to be easily
readable in plain text). html readers get the processed version of the
email, plain text readers get the raw markdown.