I have a pretty simple question: do you think the next iMac will have an SSD as standard, or are we a while away?
No, I don't. I see it from a different angle, though.
Right now I have a Nikon D90 camera (12 MP) whose RAW files are ~10MB each which means 100GB of disk space can hold ~10000 pictures.
On the other hand, Canon 5D Mark II (21MP) produces RAW files at ~25MB each. Thus, 100GB of disk space can hold ~4000 pictures. (Deviation: Guys, in 1986 the first PC at our home was a 286 with 20MB hard drive. Its space wouldn't have been enough for a single Canon 5D mark II RAW file!!!)
There is a tendency for future cameras to provide more and more megapixels which will make them produce even larger RAW files. Whether the average Joe (read: me) needs such detailed pictures is another question. The camera manufacturers like Nikon do not offer lower resolution options any more.
It is highly unlikely that space requirements will stop increasing. Besides the aforementioned example, the programs will take up more and more space. The games, especially, will require more space because of the ever increasing monitor resolutions.
Having said that, I don't think the plate HDDs are going away anytime soon. I'd give it 3 or 4 years before SSDs start to become affordable option for the low-end iMacs.