Toshiba's FTP site provides documents on their hard drives... including the yet to be announced 20GB (updated), 30GB and 40GB 1.8" Hard Drives. Toshiba's current 5GB, 10GB, and 20GB 1.8" models are used in Apple's iPod.
The files include Product Specs, Tech Notes and Agency Approval/Certificate documents for all models including the new 20GB, 30GB and 40GB drives.
Of interest, the 20GB, 30GB and 40GB have 2MB of cache. In terms of size requirements: the 30GB and 40GB are 8mm high (the same as the current 20GB drive) however, the new 20GB drive comes in at 5mm (same as the 5GB and 10GB drives).
Word of the 40GB Toshiba drive leaked out last week with a link posted on their product page. That link has subsequently been removed.
5GB iPod supplies have been dwindling, spawning much speculation of impending updates... and while hints have been dropped... no definitive information is available.