Why would they ever go to an optical disc? Especially one created and solely utilized by their biggest competitor?
Flash cards are getting such high capacities, small sizes and cheaper production that it would be foolish to use a battery draining, slow loading, bulky optical medium.
UMD - 1.8 GB - $0.50 - 1.00?
SD - 2 GB - $6.00 retail.
SDHC - 4 GB - $8.00 retail.
By the time the next Nintendo handheld is launched, those flash memory prices will be down even further and buying bulk from the manufacturer would easily bring costs down to UMD costs yet with all the benefits and none of the drawbacks.
The rEVOLution is not being televised