The new 4GB SDHC microSD cards are now becoming available at a variety of online retailers. The cards will work with the Nokia N95, but may not work with other S60 phones. MyMemory currently has stock and is selling the cards for £51.99. 4GB is enough for 4 days of music or for you to take one 5 megapixel N95 picture a day for the next 10 years!
We can confirm the N95 is compatible with the SanDisk SDHC cards. The Nokia E90 is also reportedly compatible.
Bear in mind Nokia does not officially support the 4GB cards, but then officially it doesn't support non Nokia memory cards.
We're not sure about other models, so if you have some personal experience please leave a note in the comment thread.