Designmodo's announcement for their free Flat UI (which uses Sass) mentions plans (in the discussion) to release a pay version containing LESS code. And an issue on their Flat UI gem floats a suggestion of possibly porting it to LESS.
Darthdeus' gem flat-ui-rails contains Flat UI precompiled to CSS with no LESS code.
Whether Flat UI will continue to be maintained and useful for Rails doesn't relate to Darthdeus' trivially easy to maintain gem but therefore instead to Designmodo's free Flat UI product itself. So far, BTW, their Pro version seems to be vaporware.
There is a rumor Twitter Bootstrap version 3 "Mobile first" will have a flatter UI, but that's only temporary (for development purposes) according to this article.
Web searching revealed no sign how Designmodo's Flat UI might be affected by Twitter's new "Mobile first" initiative otherwise.
Copyright (c) 2013 Mark D. Blackwell.