ilosaarirock 2005...

The festival's at 15th-17th of July. Or actually 16th-17th, as on the 15th there's just clubs, that need separate tickets.

They start selling tickets pretty late, around May-June. Just follow their website www.ilosaarirock.fi closely, they're updating it for the 2005 festival on 13th of February.

I'm not sure where to order tickets outside finland but these are the two main ticketservices in Finland, which I think also sell to other countries (I don't know which one will be selling Ilosaarirock-tickets):
www.ticketservicefinland.fi
www.tiketti.fi

Hope this helps. I've been there four times and it's IMO the best festival in Finland with very varied, mostly non-mainstream bands and great relaxed atmosphere. Of course Tuska-Festival is at the same time, and it has more metal, in case you're after just that.

Edit: Also be sure to get your tickets as soon as you can. The festival has been sold out for six consecutive years. They usually run out of tickets around midsummer (20th-23rd of June), but don't count on that.