Sandisk now seem to have released a 64GB Class 10 microSDXC (as of May 2012) , which is available for pre-order at £45 from Play.com, due on May 15th. This seems almost too cheap to be true, yet Amazon are offering the same for £50 (subject to stock).

Based on previous good reports with the (slightly more expensive) 64GB Class 6 from Sandisk I've decided to take the plunge on this one, and I will of course report on N900 compatibility as soon as it arrives...