I was thinking of the 160x cards when I wrote that, but if you look at the market there are two issues:
1 - the device bus needs to support the bandwidth.
2 - the speeds are asymmetric when reading and writing.
I know you have an iPAQ so if you've benchmarked any SD cards in that you'd see a significant difference in read and write.
So with the correct architecture I'm sure it could work - I would just hope that we get the correct architecture