My experience
I just bought a 13" 2.26 Ghz MacBook Pro, and installed an 160 GB Intel X25-M in it. It has the 1.5 SATA connection. I was really, really frustrated to learn this because I'm very careful about checking everything before I buy, but never even considered this bizarre change. However, after adding the SSD, I'm not quite as concerned.
I'm not sure how fast it could be, but this computer is meltingly fast. After a full restart (20 seconds from pressing power), opening Photoshop takes about 3 seconds (compared to about 11 before), opening Illustrator takes about 8 seconds (compared to about 29 seconds before), opening Word is about 3 seconds. Starting Windows XP in VMWare (from a suspended state) takes about 3-4 seconds (compared to maybe 10 seconds). Applications like Safari don't take any time to open: they just appear as though they were already open. Folders with lots of files with icons snap into place. I'm amazed by how often I skip a beat because something happens faster than I'm used to.
Perhaps these times would be improved with the 3.0 SATA-II interface, but I just want to let everyone know that there's still a HUGE speed boost possible by adding today's fast SSD's.
As a side note, I successfully upgraded the firmware to 8820 using my 13" MBP. It was a simple and fast process and did not affect my data.