Yes, unfortunately the 32GB microSDHC cards aren't available yet. Although there were about 20 or so pre-release units sold on eBay a few months ago, but they were defective in that only half of their space was addressable. SanDisk is mostly likely intentionally holding the 32GB release back just to milk the sales on the 16GB card.

The 32GB chips should work fine with the N810 and the N900. When I got the N810 'officially' only 8GB chips were supported (the then biggest size) but when 16GB ones were released they worked just fine without updates or anything.