Nokia just updated their Finnish page to let users know that the Lumia 900 is up for preordering in Finland.
Nokia also listed a selection of retailers and links to their product pages. I just checked with all the retailers via the links, and Gigantti shows that both the Lumia 900 and Lumia 610 will ship on May 31st. The text on Gigantti’s page shows “Toimitukset alk. 31.05.2012,” which means that shipments will begin on May 31st.