I've already replaced the boot HDD in my 2 year old laptop with an SSD, new lease of life, produces less heat so no fan kicking in all the time, everything is faster, and the battery lasts and extra hour or more (was about 2.5 hours normally, now 3.5 to 4 hours on a charge).
Cost wise, HDDs hit the $1 per GB in 2005, SSDs hit this price early to mid last year. 2005 was also the year the first 500GB HDDs came out and we did get 480/512 SSD drives last year, and 1TB drives are now on their way. So seems SSDs are where HDDs were about 7 years ago, but SSDs are moving faster, so hopefully shouldn't be 7 years to get to a 1TB SSD for £45, or 2TB for £65, maybe 2-3 years?
I think the $1 level was the critical mass point for HDDs, suddenly people were buying more and larger drives which increased demand, increased production, which brought down costs, and so on. SSDs seemed to be going the same way till the hiccup in the market last year, but hopefully is back on track again now.