So, I was messing around with the Tv Setting this AM with my XSi
A couple of shots:
Tv setting at 1/200 sec
Tv setting at 1/8 sec
Tv setting at 1/1600 sec
Tv setting at 1/4 sec
I really like the way the slower shutter speed 'softens' the water, and found after reviewing many pics (I took about 10-15 per 'view' at multiple different shutter speeds), that I needed to get down to 1/8 sec to start getting that effect.
The other thing I noticed is that earlier in the photo session, the clouds were out and it was much less bright, and even on th Tv setting, the XSi would 'limit' me to 1/200 sec. However when the clouds moved and it was quite sunny, I could go way up to 1/1600 (or higher, if I remember right).
Finally, a tripod is key for such long exposures and also using the 2 second delay is nearly mandatory as well (or way too much camera shake).