From the Interaction Page:
Hugo Nomination and Voting

Voting for the Hugo Awards takes place in two distinct stages.

During the first stage, known as the Nomination Ballot, voters can nominate up to five entries in each of the fourteen categories. You will be eligible to vote in the Nomination Ballot if you are a full attending or supporting member of Interaction as of 31 January 2005, or if you were a full attending or supporting member of the 2004 Worldcon, Noreascon 4. The Ballot itself will close on 11 March 2005, at midnight GMT. (This means that if you join Interaction during January 2005, you will still have at least 5 weeks to submit your vote, so there's no need to panic!).

During the second stage, known as the Final Ballot, voters will be able to vote for their favorites from a shortlist, typically comprising 5 entries in each category. We will be announcing the nominees for the Final Ballot at the end of March 2005, hopefully at the UK Eastercon, Paragon 2. The Final Ballot will then be distributed with Progress Report 4, which will arrive in late April 2005, with the deadline for ballot submission being 8 July 2005. The Final Ballot is only open to full attending or supporting members of Interaction. The cut-off date for joining the convention and being eligible for the Final Ballot will be confirmed in early 2005.