While we are still waiting for the European carrier O2 to receive the Lumia 900, Amazon.de has changed their listing for the Windows Phone from an expected delivery on May 31st to "In Stock".
Amazon.de is listed for 504,89€ with free delivery. The Lumia 900 is being offered in black, cyan and white models. The white model is listed at 499€. Prices do include VAT and all three models are listed as being in stock.
Meanwhile over at O2, reservations for the white Lumia 900 are still being taken with inventory expected sometime this month.
Thanks, mreck, for the tip!