I think PDA's, as they stand, are ready for the mass market. The question, in my mind, is whether or not the mass market is ready for them. The devices have been refined a lot since the beginning. Development has come from black and white, to monochrome, to color, and all the way to hi-res color. There are corporate devices, entertainment devices, consumer devices, etc. However, there are not enough people who know what can truly be done with such a device to make for a big market for PDA's. There will simply have to be something to inform people without question how useful these devices can be. From there, I think the sky is the limit.
As for electronic ink, the idea sounds great. Once it gets here, it will be interesting to see how it does at market. However, I think it will have to be in color to compete with today's LCD devices.