Yes, rails has simple_format which does exactly what you are looking for, and slightly better since it also adds paragraph tags. See
http://api.rubyonrails.org/classes/ActionView/Helpers/TextHelper.html#method-i-simple_format
Example:
simple_format(mystring)
Note that simple_format allows basic HTML tags, but also passes text through sanitize which removes all scripts, so it should be safe for user input.