Right now $120 buys you a pretty good (Seagate Barracuda 7200.11, WD Caviar Black, etc.) 1TB standard hard drive. Same $120 gets you a pretty good (OCZ Vertez, Patriot Warp II, etc.) 32GB SSD. Now, as long as I can remember, something in the $100-$120 range was what you paid for a pretty good hard drive, so lets assume that this remains the price point companies are trying to hit for mainstream consumers.
According to Kryder's law (a formulation similar to Moore's law hypothesized by Mark Kryder, PhD from Cal Tech and founder of Carnegie Melon University's Data Storage Systems Center) the data capacity of a given magnetic drive doubles every 12 months. If that trend holds true for SSDs, they should hit the 1TB mark at about $100+ in about 5 years.
Now, another version of Moore's law says that RAM capacity doubles every 18 months, basically in sync with processor transistor count. If we say that SSD technology is more similar to RAM technology, that would mean the 1TB mark is 7.5 years away.
Split the difference and its 6 years, 3 months. That's a long time to wait. Furthermore, if we assume HDD capacity will be growing at the same rate it has been since the 1950's those 1TB SSDs will be competing against HDDs with over 75TB capacity.
Alternative Scenario: The WD VelociRaptor currently has a max capacity of 300GB at a cost of about $220. Assuming that 300GB is a reasonable level of storage that most people will be willing to accept in order for a significant performance boost than we need only calculate when SSDs will reach the 300GB for $220 (or 150GB for $180 if we look at the previous generation) level. At those price points, current SSDs offer 128GB and 64GB storage respectively. Therefore, a reasonable prediction would be that SSDs will reach current high-performance HDD capacities and prices in 18 to 23 months.
Therefore, I would suggest that purchasers look at SSDs being competition for current high performance HDDs like the VelociRaptor, not mass storage 7200rpm drives. SSDs will most likely never replace those HDDs. Remember, most companies still use tape drives for large data storage or backup and HDDs, rather than replacing tape, have become a complementary technology. SSDs will most likely go the same route, supplementing HDDs and replacing them in certain applications but not spelling the complete end of spinning platters by any means.