According to Ballot Access News [1], the Libertarian Party of Iowa now has 58 registered Libertarians. In the past, Iowa voters were not given the choice of registering as a Libertarian or Green, but that changed after a lawsuit last year.
To view a complete state-by-state list of Libertarian Party voter registration click here [2].
Links:
[1] http://www.ballot-access.org/2008/02/19/for-first-time-ever-iowa-has-registration-data-for-unqualified-parties/
[2] http://users.rcn.com/k.brennan/LibertarianVoterRegistration.html