Over at the german spoken Telefon-Treff forums this photo emerged. It’s from ”John”.That picture shows that 4GB microSDHC works in the E90. Here a shop where you can buy this card.
They are sold out But at Mobymemory (UK) they are still on stock and not expensive at all (£39.99 incl. SanDisk USB reader): http://www.mobymemory.com/phone-memory/Nokia/E90-memory-cards.asp