As far as drafting Ricky, I can't see myself pulling the trigger before the 8th round in my 12 team league. I have a few reasons for this. First, as mentioned by previous posters, I think they'll be trailing in enough games thanks to their defense. They weren't that good last year, and trading away Taylor (even though it makes sense for the long term) doesn't make them any better. They play some good offenses in their first 5 games, so even if RW is slated to get the bulk of the carries early on, those won't be games where the Dolphins will be in a position to run out the clock.
The second reason I won't be drafting Ricky higher than the 8th is that I don't believe the "Brown could be traded" talk. It makes no sense because even if Miami decided he wasn't their guy, his value could hardly be lower at this point. He's coming back from an ACL injury, now he's got a bad thumb. If Parcells and Co. want to deal him, better for everyone involved to do so after Brown has gotten healthy and flashed some of the form he displayed in 2007 before his injury. As a result, I think RW and Brown will be sharing the load this year.
The third reason is, it's Ricky Williams. Yes, I remember how talented he was. He helped me win my main money league in 2002. When his head was in the game and he was healthy, the guy was a beast. But since the 2003 season, he's played in all of 13 NFL games. Why? Attitude and injury. His head appears to be on straight, but how long will that last? And what about his health? 4 of his 7 NFL seasons saw him play 12 games or fewer and his one season in the CFL was injury-plagued as well.
I think Ricky could do well this year - Brown certainly could get hurt more severely and miss time, and perhaps the addition of Pennington will make the Dolphin offense more productive (though I don't think it will stretch the field enough to keep opposing safeties from creeping up closer to play the run). Perhaps Sparano and Co. figure out a way for the defense to improve on its #23 ranking from a year ago and keep things close enough to be able to run in the second half of games. But with the conditions that exist now, I think Williams is too risky to spend anything earlier than an 8th rounder for me.