3GSM
Here's the world's highest capacity MicroSD card, according to SanDisk that is. The solid-state memory specialist today announced the product, though it would only say the 4GB unit will ship "later in 2007". It hasn't worked out the pricing yet, either.
Still, the card meets the key SDHC criteria: a capacity of more than 2GB and adherence to the SD 2.0 specification. The SanDisk MicroSD card is Speed Class 2 - it offers a minimum data transfer rate of 2MBps.