The current generation of SSD's are smaller and slower than regular disks in the same form factor, but this is starting to change as products like this Samsung 256GB SSD reach the market. It has a 200 Mbyte/s read speed and 160Mbyte/s sequential write speed, in a standard 2.5" disk form factor.
So the sequence goes like this:
1) SSD's will be faster for random access (already happened)
2) SSD's will be faster for sequential access (coming later this year)
3) SSD's will be higher capacity (maybe next year?)
4) SSD's will be lower cost per GB (when production volumes ramp up)
The advantage in random access and reliability (due to no moving parts) means that relatively fewer SSDs are needed than spinning rust disks to provide the same availability and performance, so the end user cost per configured GB should switch in favor of SSDs earlier.