Many likely scenarios:
5% - FWP plays goes lights-out again vs. Houston and plays solid vs. New England. Cowher annoints him the 'starter' (with the Bus still taking SOME goal-line carries), he finishes the year with 1600 combined yards and 10-15 TDs. This is true RB1 land. Locked as PITs starter for a few years to come at the very least.
25% - FWP plays well for the next few weeks, good enough to hold of Staley and receive almost all of the carries between the 20's along with a few others. Bettis still vultures his share and Staley still gets some love. 1200 combined yards and 7-10 TDs. You're talking about an average RB2 at this level of production. Likely the PIT starting RB in 2006, so solid dynasty value.
40% - FWP plays OK, but when Staley comes back they split time, say 2/1. Along with Bettis, that gives FWP maybe 150 carries, 800 combined yards, 4-6 TDs. Suitable RB3. Possible dynasty value, but a strong possibility that PIT looks elsewhere at RB in 2006.
20% - FWP plays fine next week, but struggles vs. NE. Staley comes back and we see a true RBBC. 100 carries, 500 combined yards, 2-5 TDs. Still worth a roster spot in redraft, DEFINATELY not the long-term answer at RB in Pittsburgh, so dynasty value is null.
10% - FWP is exposed as a fraud during the NE, or maybe even Houston game. Duce comes back healthy and FWP is limited to 3-5 carries a game. We all have a beer and laugh about him some day. No dynasty value, likely not even worth a redraft spot.
All of us, experts down to guppies, can't say with any certainty where in this range he'll land. Above is just my gut feel.